I have used airbnb a few times.
It is a step away from the world of miles, points and hotel status, but for longer stays (e.g. several days to several weeks) it can be good. A furnished apartment/basement/granny flat is more "homely" than hotel rooms. It is especially good for groups and families as paying for several hotel rooms can add up quickly. Naturally there is a range of quality and price points, but the site tells you most of what you need to know.
Also, often times the hosts can give you invaluable local knowledge. Purely by chance we stayed with a guy who shared a dorm room with the founder of airbnb. He was super friendly and we shared a meal or two. He picked us up from the airport and showed us a few places around town to check out.
A very different style of travel, but something which has already started to take off, and could grow even more. Also a great way to rent out apartments, etc on a short term basis. Potentially quite lucrative if you can establish a steady stream of tenants.