I've used airbnb elsewhere in Spain and it was brilliant. You do have to use your instinct when you look at a listings, e.g. if the person comes across in their profile as a bit odd, lazy, or strict / unreasonable then go with your initial feelings on them and avoid. Be a bit wary of a listing in any locality that's much cheaper than the rest: there's going to be a reason why.
You have to be flexible and communicate well with the host, as (like Rebekah says) a person with a busy life, who is renting a room, cannot sit indoors all day waiting for you to turn up, so in my experience it can involve some texting or phoning on the day to co-ordinate arrival, which if your Spanish is very minimal, could be a challenge. Also bear in mind that they will review you, as well as you reviewing them, so if you were really unhappy, you'd have to be careful how you handled it.
Mainly, I've met some lovely people doing it, been invited to have wine in their kitchen, fabulous big chats etc. It's just very nice being in a Spanish home.