i've only used it once about 2 years ago and i had a great experience i was staying in a small basement apartment in san Francisco owned by a guy called glenn. he was the most helpful and caring person gave me some great advice about places to eat or visit.
with airbnb i think its more a case of reading the reviews of someone and seeing if there a good host as unlike a hotel where you hope for some form of consistency between them your dealing with individual people and there homes etc but they can make a trip that bit more personal and special with the knowledge of a local at your fingertips.