We installed the 27" version of the Kitchenaid, the KEMS378SSS, in our kitchen remodel a few months ago. I've never had a convection microwave before, but have used the convection feature for frozen goods like pizza. There it seemed to perform fine. The controls take care of converting a normal recipe to convection temps and times. However, I've only used the dedicated oven for true baking like roasts and bread.

It is initially a bit inconvenient figuring out which trays or sheets to use with the upper oven, as it comes with a bunch of accessories for microwaving and others for convection, and am not sure what stuff for the normal oven would work.

The biggest concern I researched here beforehand was that if the microwave fails, repairing it is really costly, and you can't just replace the microwave unit. However, just a few months in, my real concern is the robustness of everything else. First, when the power was cycled on, the unit gives a Door Latch Not Operating error code. Can't clear it with the controls, but figured out if you open the door it will clear. Recently I cooked a pizza in the lower oven, and after it cooled there was another error to replace something - could not clear this one at all except to reset the breaker. There's a long thread about failures when self-cleaning because of overheating. The unit blows a lot of hot air out during cooking - apparently this is also to prevent the electronics from overheating. I can understand this after roasting something for an hour, but sometime microwaving leftovers for just two minutes causes the fan to blow for another five minutes! The oven won't work at all if there is an error code showing, so does not inspire confidence for the long run.

This one is a no brainer in my humble opinion....
Go with the combination of a GE Advantium 120 with a GE Wall Oven....

That combination gives you the flexibility of using them as:
-- two ovens
-- an oven and a microwave
-- an oven and a warming drawer
-- an oven and a speed cooking oven

You get it all! I personally installed the 27" wall combo and have no other ovens or microwaves in the house. I cooked the Holiday meal for 30 with just that combination.
Turkey in the wall oven, two sides in the Advantium.
Good Luck!

I was considering the KA double oven until I read so many negative things about them on these boards. I went with the GE double and GE refrigerator since they are next to each other. On the otherside of the kitchen, I went with the KA dishwasher and KA cooktop.
I'm still waiting for them to arrive. Seems the GE double oven is on back order. I place my order 4-23.