We've only done it for a month in the summer--twice. Once a few years ago we had a 17 year old French girl and she was great. We got her when I saw an ad in the paper that host families were needed for French students. I think the organization was LEC.
Two years ago we had two French brothers whose uncle had stayed in our neighborhood when he was a teenager. The families kept in touch for 25 years and we got them in a roundabout way. They were the same ages as our two boys and they got along okay. I'd say they were definitely harder.
We have four kids and both times we got kids who were from bigger families (the girl was the youngest of 4 and the boys were the oldest two of four). I'd say just make sure you don't get an only child with your "busy" household or it may be a little too much! I know we looked for someone from a larger family and a little older (better English skills)--there were a lot of 15 year old only children and I think they wouldn't have liked the craziness of our house.
This summer the middle two French boys are coming to visit for 2-3 weeks.
The only thing that I don't like about it is that I feel like I can't let loose and be a biotch in front of them. But I have a couple friends who've told me that when they had exchange students when they were in HS, their moms got over that after a couple of months.