Using AirBnB for the first time -- currently staying at a great apartment in Manhattan. My girlfriend and I are both quite happy! Both of us, and the apartment owner, are first-time AirBnB'ers. It _can_ work, I believe, but you've got to vet things carefully.
We had to work around an understandable but (I think) misguided AirBnB restriction that makes it hard to get phone numbers in to messages -- it's so important to be able to talk to someone to understand who you're dealing with!
From what I've seen, with most of them you either get your own bedroom or the entire house/apartment. It's really no different from a typical vacation rental, except the owner is possibly crashing at a friend's house or a motel during your stay.
But the idea of sleeping on some stranger's couch doesn't appeal at all (plus the unknowns like cleanliness, security, etc)