I have the Gaggenau ovens and I love them. The big difference between the Gagg and the other manufacturers is that the Gagg doesn't have the auto settings..... you have to figure out the time, temperature and steam levels yourself. It's a bit of a learning curve, but not that difficult once you get over the terror of the new appliance (not unlike the terror that people with a new IP or sous vide circulator feel). And there is a lot more info now than there was even 3 years ago! I haven't used my broiler more than a few times in the 3 plus years that I've had the oven. I have the broiler in my regular oven, and I really don't need 2. The other thing to consider is cleaning. The biggest peeve that I've heard is that cleaning behind the broiler in the Miele is a pain. The Gaggenau broiler is behind glass, so it's easy peasy (plus Gagg has a self cleaning feature). I don't think the Wolf is the worst choice.

I had the plumbed Gaggenau combi for almost 15 yrs in my old house. I absolutely adored it but prayed every day that it wouldn’t fail since we had no local service. If you have good local service and don’t mind spending the extra cash For the oven....go for it. Definitely.

new house..in one year now with my new Wolf combi. Different learning curve...simpler I think, and I love it just as much as the Gagg. It doesn’t bother me to fill the water reservoir or wipe a little water out of the bottom of the oven. Couple of paper towels and it’s done. No regrets. Good Wolf local service here for us so it was a no-brainer. It’s just as versatile and suits my needs. If you are a gourmet cook ..you may be dissatisfied compared to the Gagg. I don’t know since I’m not a gourmet cook, lol.

Before you commit to the Wolf M, read some of the discussions about the M on this forum. Also read about the issues Wolf has with their blue enamel chipping which has been going on for 10 years and has been reported with the M. The M was made with a replaceable bottom to make it easier to repair.

Most people here mix and match because no one manufacture is best at everything.

For the combi oven, I would read the manuals. Gaggenau has an excellent chart with which humidities are used for different parts of baking and roasting.

As an aside the griddle is nothing special save your self $$$and buy a griddle that fits over 2 burners for $25 one of those has stood the test of time in my kitchen for eons. IMO Wolf is overrated and over priced and no one needs a 48” range unless you cook for 20 everyday . The venting alone will cost as much as the range. I have used steam ovens in commercial kitchens the last one was being repaired more that we used it, Think about how you cook applainces need to be bought for how you cook not because they all “match” good old plain stainless from any maker will match from another maker Function is the first item of business in a kitchen all the pretty stuff in secondary. You need an actul kitchen designer to help with all these decisions not a cabinet salesperson but an real KD I sit down with my clients for a long talk about how they hoestly use appliances , I run a catering biz from my kitchen I have a 36” 6 burner all gas range Bertazzoni, Fridgeadair all fridge/ all freezer columns with a built in kit , a Bosch DW a MW and single induction burner I use for really delicate sauces and double KA wall ovens too. I can cook for as many as I need in this kitchen so my advice is really think about how much money you are spending to impress people and how much you really need. A really good idea is to find a appliance show room that does demos that way you get what works for you. BTW I have actully done a banguette for 345 people out of my kitchen

I don't think there is any guide to the best combi oven - most of them are fairly similar, and the major differences are whether they are plumbed or nonplumbed. I have an older Viking, and found it hard to understand at first, but after I while I learned the various functions. Some ovens have a lot of automatic programs for different food, others like the Gagg are much easier , IMO , to understand, you set the temp and humidity directly. In any event, for most of its use, you will likely use it as a straight convection oven, because it preheats in a fraction of the time of a full sized oven, and are plenty large inside for most dinners. For things that are plain steam - like shrimp or veggies, that will work out easily in most ovens. I do use the combi mode some, but no where near as much as straight convection, but I think what ever one you get you will figure out how to work it to fit your cooking.

I have copied my post from another thread, I have done a bunch of research but haven't taken delivery yet. The original poster I was replying to was concerned about cleaning the units so it has some more detail about that specific activity than you may care for... also I wouldn't get the wolf oven. I agree with wekick... there are too many horror stories with the enamel chipping that it just isn't worth the risk with such an expensive appliance.

Here is what I have learned, I hope it will help you.  I will include some thoughts on the Gaggenau model as well as I have done research and had a cooking demo on it as well.

The Wolf:

Just went and looked at it again today...I have a problem :)
It's my least favorite of the ovens but with the discount on offer right now it's also the cheapest.

2 year standard warranty.
No broiler or bake element, just convection.
No manual control over steam levels, just two modes 'Covenction Steam' and 'Convenction Humid'. Humid' doesn't emit steam but seals the oven cavity to preserve moisture.
No option for a plumbed unit.
Oven size: 1.8 cubic feet

In terms of cleaning, I do feel like it is on the easier end of the spectrum.  No self clean mode but, it recommends that you set the steamer at 175f for an hour to loosen grime and then wipe down.  As there are no other elements it's really just cleaning a stainless steel basin. There are a lot less cleaning rules in the manual for this one than in the others.
One other thing to note, I find the doors on the Wolf to be the heaviest.
I do like how it looks aesthetically ( contemporary, retractable handle) and how it is entirely flush with your cabinetry.  The other options all stick out to varying degrees due to the handles.

The Miele:

Very nice oven.

As Kim mentioned it comes in two sizes the XL (48L, 1.7 cu ft) and the XXL (68L, 2.4 cu ft). With the XXL you are getting quite a bit more oven space for a similar price, the only tradeoff being preheat time.
In addition to the convection fan it has a covered baking element in the bottom and an exposed broiler element on the top. Some people on here have complained that the broiler element gets dirty and it hard to clean.  I was at Miele the other day (I have a problem!) and after insulting with the manager it looks like a screw driver is needed to get a good clean above it.  Taking out the element entirely does not seem possible.  I am going back to talk to the chef there next week.

There are a lot more rules about which surfaces to clean with what in the manual. The cleaning section is like 5 pages vs wolf 2. The 'PerfectClean' thing is a non stick type enamel coating on the interior that is supposed to make the recommend cleaning after each use easier; not a self clean mode.

In short I'd say hardest to clean of then all.

Right now they are offering 10% off or a 10 year warranty. I think that warranty is very, very nice and may tip the scales for me alone.  One thing a lot of people here seem to repeat is that these CSOs are complicated machines, that will probably need some service at assume point. It would be nice if that service was free for 10 years.
Best steam control, as others have mentioned. You can move the humidity in 1% increments although after reading about the steam generation methods on all these ovens I am skeptical that it could have that kind of accuracy.  But definitely most adjustable of them all.
I like the looks.  The ergonomic handle with the swivel is very smart design.  I don't love the control system compared to the others.

Thermador:

I will be honest and say this is one that I did not research as deep as the others.  I have seen it in person twice.
It has the largest capacity of all the ovens at 79L or 2.8 cu ft.
It terms of steam control it has a mode where you can choose a temperature and either low, medium or high steam assist.  Definitely a little vague.

The Thermador has another type of coating called ecolytic. It says it is a matte ceramic material that is non porus.  It can be refreshed with their high heat auto ecolytic clean mode.  I have not seen this in action or heard anyone's experiences with this but know that people are wary of high heat self cleans these days.

With the exposed broiler I imagine some of the same cleaning concerns as the Miele. The cleaning section in the manual is detailed, but the tldr is clean with soapy water.
2 year warranty.
Styling and control scheme is not for me but not bad at all.

Gaggenau:

This is the oven I thought I wanted at the beginning and will probably still get.  It looks really sharp.  The controls are great. No handle at all, which is really nice.  Also the only one of the bunch that has a side open hinge. Could be good to limit bending.

It is the most costly of them all and there are no o bundles or deals on the near horizon (Their terrible promotion ends at the end of the year).
Oven size is 1.7 cu ft or 48 L.

Steam control has increments at 0, 30, 60, 80 and 100%.
One thing of note, that I think Sheri mentioned is that this oven does not have the hand holding modes that the others do. You have to set the heat and humidity yourself. Appeals to some not others.
2 year warranty unless you buy 5 Gaggenau appliances, then it is 5 years.

It has a broiler under glass ceramic so it doesn't get dirty. No bake element on the bottom.

For you, perhaps the most valuable part, it has the only real self cleaning/descaling mode that I have seen.  This is the one that you plug the cartridge into the roof of the oven and leave it to do it's thing for a few hours.  Seems pretty simple. Not a high heat clean.  Looks kinda like it turns into a dishwasher.

For what it's worth I have narrowed down to Miele XXL or Gaggenau.  
Hope you love whatever you get!

When the appliances are in the kitchen NO ONE, and I mean NO ONE including you will notice if all the handles on all the appliances are an exact match or if they match at all.

I can't see spending extra money for example for the Wolf drawer microwave when all the microwaves are made by Sharp and are just rebranded with another name.

And I would never get the Wolf ovens because of the chipping issue.

Buy the best appliance for how you use it.

We have had similar issues in terms of matching appliances and are going for very similar things.

The Miele XXL oven is much, much more controllable and versatile than the Wolf, and so I crossed out the Wolf Combi right off the bat, plus, PLUMBED. IS. HUGE. Gaggenau interior is fairly small and the look is a little to overtly modern for me, plus service is a bear and control is not as good as Miele.

We are doing a Miele stack with XXL Combi, Vac drawer and Warmer drawer. We are also getting a handle-less Wolf 30" convection oven as our main oven, mounted low, as we are doing a 60" BlueStar RNB rangetop with a 24" griddle.

We are mixing and matching elsewhere and think the aesthetics will be fine - all the SubZero (fridge,freezer, ice maker, beverage center) will be paneled, and so will the two dishwashers (Miele Crystal and Cove).