We are former military, so we sometimes didn't have much of a choice, especially if we weren't stationed close to my family.
I only had one incident where it ended in complete disaster. My day care provider decided to take the summer off (her hubby worked with mine). So we asked our other neighbor if their daugher, 15, wouldn't mind babysitting. She seamed so mature, but this task was too much for her.
We had explained everything to her and felt really secure knowing that hubby worked 5 minutes away and her mother was right next door also.
One day maybe a week after we hired her my hubby calls me in the middle of the day from home! The babysitter had ended up watching TV and our little boy had locked himself into the bathroom - she didn't notice at all!!!
When she finally realized a couple of hours later that she hadn't seen or heard from him and searched the apartment she realized that he was behind the bathroom door and she didn't know how to unlock it from the other side. So she called hubby and he came home. That was the end of her babysitting duty. Our little one then ended up spending the summer with my family where I knew my mother would keep a good eye on him.
Other than that I have had lots of friends that would sit him for the night or he would use the programs available through the military.
When he was older and we lived in Japan a friend of mine had a teenage daugther that would watch him occassionally, but by then he was 9 years old and knew how to use the phone and we had neighbors and her parents that would check up on them frequently.
Personally I would never entrust a small child to a teenager unless she would babysit in her own home and mom would be right there to help in case of a crisis.
Hope this has helped you!