I have been pointed to airbnb on several occasions. Although the idea behind that website is really nice, I never actually made a reservation through airbnb. Some times the offers are too good to be true and some times the responses you get from the hosts are not really building confidence. I almost always end up back on the good old booking.com and tripadvisor.com
My main concern with a booking through airbnb is that you arrive at your destination and the whole booking seems to be a scam and then you are stuck in an unknown city with your only refuge being a 4 star expensive hotel since all other accommodation options at that time have no vacancies.
In the dutch media there is a lot of attention to services like airbnb. Rental agencies and banks seem to actively monitor airbnb to find people not applying to their rental and mortgage agreements. A related question shows that legal issues apply in other places as well.
I really love initiatives like airbnb, but I always seem to end up using conventional aggregators where there is someone to blame if things go wrong.