I've done it before. I have 3 cats and it's significantly cheaper to get someone to visit once a day than put all 3 in a cattery. Personal recommendation is the best way to find them. As much as I have to suggest it, FaceAche is a good way to find someone. I happen to do dog-related sports, so know quite a lot of dog trainers, dog walkers etc. One of the dog walkers happens to walk dogs in the area, so would pop in to see the cats on her rounds.
Things to look for - if it is their job, they will have insurance, ask for details of emergency contact & vet details. There's still a trust issue as you need to give them keys, but if they go to a dozen houses, and you know exactly who they are, and have copies of their paperwork, the risk is much reduced.