As @Pattidevans says, read the reviews carefully, checking dates, as you would with anything else. The ones we have stayed in have been lovely. Clean, comfortable and just what we've wanted, usually self catering. Younger daughter always stays in them wherever she goes around the world and she's never had a bad one either. I suppose its the same with wherever anyone chooses to stay, whether its hotel, b&b, self catering, you will always get good and bad.I’m a bit like that too. Although never used them but you hear such horror stories.
We sometimes go with booking.com as well, simply because you don't need to pay deposits and you can cancel last minute without losing any money.