We had 2 14yo French girls for 2 weeks three years ago and 4 14yo French boys for 4 nights two years ago.
We found the girls to be fairly self contained but they didn't interact with our children at all and had very limited social skills. They spent most of their time that they weren't at language school on the PC on msn messanger and so from that point of view they were 'easy'. They didn't engage with us though despite trying, even one time there was only them and me eating dinner at the table as DH and DC were out, they spoke to each other in French and ignored me. I could only make so much conversation! (We were told not to speak French to them, so at the beginning I told them I could speak French so if they had any trouble they could speak French but only then. At the end of the holiday I told them I was aware of them swearing constantly in French all their stay, not about us thankfully, and told them I actually speak fluent French, it was so amusing to see their faces!!). I don't know what I'd have done if ty'd been unpleasant about us though!!
We also found that having strangers in te house for 2 weeks was hard work. I think we were paid £400 for the two weeks for both girls. Yes you do make some money but actually the petrol and extra food ate into that. And I paid my DC some money for sharing a room for 2 weeks. We had ok girls though, one family had a nightmare 15yo who lied, smoked and stole drink and was generally awful!
The 4 boys for a shorter period was better we found. They interacted more and played with our children in the garden etc (well 2 of the 4 did), they had better social skills and the boys enjoyed their company. Pay was more pppn as it was short term but again we didn't make much really. I also paid my DC to use the spare bedrooms ( we have 2 spare bedrooms but they are a double and a single whereas my DC each have a room with bunk beds).
We didn't do it last year as DH worked abroad for the whole of July when they were coming over and I didn't want to do it on my own.
Interestingly my boys (now 11 and 8) have now asked to have foreign students again not having mentioned it at all in te last year or so. I think I must be off the mailing list now though so will get in contact. My DS1 is going to senior school this year and learning German do it would e hang to get German students over but I think they tend to be French or Spanish round here.
Do it for the experience but not so much for the money, as food and petrol/running around after them does take its toll.