We've had combo microwaves for years and wouldn't be without one.
The most useful ones have a grill, fan oven and microwave all of which can be used independently or in any combination. Ours have been Panasonics, so can't answer for the features of others, but what we have found it especially good at are:
- It's programmable, with 3 steps, including delay. I often use it to defrost & heat in one go, starting & finishing with a short burst at full power with a longer bit at medium in the middle. Ditto things like oven chips or scampi: use oven/microwave to heat them and get them started, then switch to oven/grill to crisp them up. Works a treat;
- Baked potatoes are especially good. Soft interiors and crispy skins. Ours has a special programme where you just enter the weight and it does the rest, but all it is is a combination of fan oven, grill and microwave. Perfect large baked potato in 13 minutes. The same applies to pizzas.
- Cooking & heating small quantities which wouldn't justify putting on a full oven.
The main drawback, I suppose (apart from cost) is that they tend to be larger than a basic microwave oven. If your new kitchen also includes a new cooker, there are one or two suppliers who put these features into the top oven of a double. Alternatively, go for a cheaper single oven cooker and a built-in combo microwave.
To cut a long story short: cost apart, a combo oven is a no-brainer.