Just back from my 2nd Airbnb holiday, I was in Madrid. The booking process was very smooth. The apartment I booked was exactly as described and pictured, there were no surprises, and the owner was there to greet me on the first and last days.
As a single traveller, I can make considerable financial savings by using Airbnb (a couple of years ago, I stayed in Boston for 1 week for just over €500 - I couldn't get a decent hotel room for that).
As galwegian44 said, read the reviews carefully - it's not quite as bad as tripadvisor, where people grumble about the slightest thing, so you will get a feel for what's right and what's not with any prospective property.
I'd be happy to use them again.