I've used airbnb three times so far (in Pittsburgh, Bangkok, and Istanbul). Generally positive experiences, especially in Istanbul where the apartment we chose was being renovated so we got a big free upgrade.
In Bangkok, hotels are so cheap these days that I would have been better off with Priceline or Expedia, but my airbnb apartment did give me a sense of what housing is like for working class people there (i.e., not great).