Mikepier wrote:

AFAIK, most over the range microwaves are 300CFM's. Most standard vent
hoods are also 300 CFM's but the higher end ones can be 400CFM's and
higher.

You can now buy 400 CFM microwave/hood combinations. I just put one in a
rental unit that had a 25 year old combo with a weak fan. The new one
cost about $250 on sale during a "friends and family sale," see:
"http://www.sears.com/shc/s/p_10153_12605_02280082000P"

You can buy standalone hoods that are well over 800 CFM, but the
"standard" cheap hoods are very weak, often 200-300 CFM.

I just did renovated my kitchen and my new GE microwave hood does a
good job taking out smoke and smell. Keep in mind a hood is only as
good as the ductwork it's connected to, so make sure the runs are
short with not a lot of bends.

They work well as long as you're not doing a lot of high heat stir fry
that generates a lot of greasy smoke. However as others have pointed
out, they do get the outside of the microwave dirty, requiring more
frequent cleaning. The filters keep the vent work behind the microwave
from getting clogged up.

We replaced the old built-in double oven, that was in the house when we
bought it, with a microwave on top, oven on the bottom. This saved space
versus a standalone microwave, and since we rarely used two ovens at
once anyway, was not much of a sacrifice.

Unless the original poster have a really small kitchen, he should avoid
the combination unit, unless he doesn't do a lot of high temperature,
high fat cooking.