We had a combo unit, and one did experience an early demise. We were a little annoyed with the extra expense of buying another combo unit however what we discovered is that would have been a deal compared to what actually happened. Nothing on the market at that time would fit the cabinet we had, so we needed to replace that tall cabinet. Matching existing cabinetry is difficult, and we were getting no help there. In the end, the dead appliance morphed into a complete kitchen remodel. This time, we're doing our ovens as separate units, and we're making sure that the oven stack is made with separate components, so that they could be unassembled and remade in another size. What we had before was one tall cabinet, floor to ceiling, that was impossible to alter without destroying it because of the way it was constructed.

There would be little difference in looks. The Bosch combo units don't even look that sleek anyway with their separate control panels, at least compared to some of the other offerings out there. The photo you posted is not of a combo unit, it is of a 24" Miele speed/steam oven with a trim kit stacked on top of a 30" Miele convection oven.

The speed oven part of the HBL8751 would be more functional than the HMC80151UC because the combo unit has a 240V speed oven. The 80151 is only 120V which means longer cooking times and inferior quality results on the speed mode. To compare apples to apples, you'd need to stack the HMC80251UC speed oven, which means an extra 240V circuit needed and increased costs.

Yes, the possibility of one unit failing early is a notable downside to combo units.

The bosch HBL8751 is actually 2 separate units with a shared power supply. In fact, they ship in 2 separate boxes and are attached via mounting brackets in the field. If the speed oven or microwave dies down the road, you could easily replace one or the other. The difference between the 120V and 240V in performance is not enormous (from Bosch's mouths not ours).

That's good to know, I didn't realize that. In that case I don't really see any reason not to get the combo unit, since installation should be easier and only 1 circuit is needed.

I was basing my comment on the difference between 120 and 240 on the Advantium, where there is definitely a difference in performance. 120V speed ovens are limited in the number of things they can do at once, so they are forced to rely on microwaves more to maintain the "speed" part of it, which leads to worse results.

Joe Henderson, thank you for offering that info. I have a question regarding the separate units with shared power. I'd really like a combo unit with steam oven but I also want a side swing oven. Unfortunately, I only have one 240 power supply and can't add another. Is there any way to combine the two individual units but have a shared power--how it's apparently done from Bosch?

Thank you for the comments. I was not aware of the different options for 120V vs. 240V so I will have to look into that.

I'm still slightly confused though about the ability to replace just the microwave at some point down the road if I get the combo. Could I, for instance, just add in a new non-Bosch microwave into the slot with just minor cosmetic fix-ups or am I going to have to replace the whole combo unit?

The combination units only come in drop down door models for some odd reason. The HSLP751 is the combo with steam and that is a single power supply unit with modular pieces (so if the steam fails, the oven is still good). That being said, speak with an electrician, perhaps its ok to split a single 50A line into 2 junction boxes to power the 2 units. Then you could get the HSLP451 and the HBLP451R/L and mount them on top of each other.

As for replacing the microwave, you would have to use a bosch unit to do the replacement. The combo unit will not have any provision to power or support a non bosch unit down the road.