My wife & a friend of ours have used an AGA Interspiro (my wife's). Here's what I've heard from them:
1.) Fogging up is basically a non-issue with it.
2.) Don't have to fool with clearing it much, easier.
3.) That particular mask is somewhat harder to equalize in vs. a regular scuba mask, but doable.
4.) They go through air fast than with a standard scuba mask.
5.) The 2'nd stage doesn't disconnect, so if you had to use a buddy's octo., you'd have to pull off your FFM. I've heard there's some FFM brand/model out there where you can disconnect the second second stage so you can receive a buddy's octo. without having to take the whole thing off; I'd check into that and see if it's worthwhile before I bought one now.
6.) I have a beard so I've never tried it. I don't know how good a seal you'd get with facial hair a factor.
7.) It is a bit positively bouyant; that $100 lead weight might actually be worth buying (for this brand/model).
8.) The ability to breathe through your nose is nice.
My wife prefers diving it for general usage.
Richard.