I've done it twice and had bad experiences both times, but that's a pretty rare thing and I'd still be doing it right now if I had more time.
I was a big brother to a 6th grade kid with "ADHD" (according to someone...his mom was a little strange and I bet she really pushed for the drugs) for almost two semesters. We didn't really do much....we just hung out and played. Once I went to watch him graduate from one belt to another in his karate class, and he saw I was there and had a big weird smile from it. We went to the mall a few times around Christmas (this was just a year ago) and played with all of the stuff without buying anything. One day I went to his house and he was surprised to see me. He asked me, "Didn't you hear? You're not my big brother anymore." I didn't really know what was going on. Nobody told me anything about it. I just left because I figured his mom wouldn't want me around that day. Well, come to find out, his mom called it off and nobody really knew why. The lady at Big Brothers was actually mad at the mom for doing it, which was pretty funny because she was this nice old lady who you wouldn't think would get mad about anything. The lady seemed to think the mom was jealous of me, but I was never really sure about that theory.
After that I decided to try the whole thing again, and I got set up with a kid in elementary school. On our first meeting I went to his house and his dad said he spent the night at his friend's house the night before and forgot all about it. I kept calling after that and he wouldn't be there or he would be busy. I finally decided that this kid really didn't want to be in Big Brothers after all. I think his parents pushed him into it because his sister was doing it and she really liked it.
Ha....I know it all sounds bad, but I'd do it all over again. Just remember one thing: you want to be cool and be the kid's friend, but in certain situations you have to throw all of that out the window and just plain be an ******* about certain things. Don't be afraid to be tough in certain situations. That was the hardest part for me. I wanted to be a friend, but kids will walk all over you if you let them.