I've done it 3 times - a German boy, a Norwegian girl, and another German boy.
First was a bust - my kids were young (6 & 7), so we asked for a kid with younger siblings. His siblings were 14 & 15. Not much help. His primary goal was to be an airline pilot, and he wanted accentless English. He had no interest in being a part of the family, and was under the (grossly mistaken) impression that we were being paid to house him, and therefore we were his "staff".
Four years later we tried again. This one went better - and she genuinely enjoyed being with the family, and having little brothers. We lost contact after she went back home, though.
Third time's a charm. Our sons were in 8th, 11th & 12th; our import was also a senior. He was not your stereotypical German; he was extroverted and had a very goofy sense of humor. He got along well with our kids, and formed his own friendships once he got into school here. We've maintained a close relationship with him ever since; he has come back to visit a half-dozen times, and we have hooked up with him & his family several times in our overseas travels.
My advice would be to research the sponsoring group. We went with two different agencies...one was MUCH better than the other. If possible, correspond with the student before they come over. Make sure they know your expectations of the experience, and make sure they know you are NOT getting paid to feed, house & transport them.

We found that having a student that already had their international drivers' license made life much easier - they were able to drive in Iowa fairly easily.
It can be a really fantastic experience or a really bad one. Most of it depends on the efforts of the parties involved.