My sister is some sort of area coordinator for AFS south of Boston, so she's had exchange students for years. Most of her experiences have been good ones, but she had a couple students who were spoiled brats. On the whole, it has been a wonderful experience for her whole family. All three of my sister's kids have been to Thailand with AFS.
As far as I've seen, AFS seems to be the best of the exchange student organizations - possibly because it's so heavily staffed with volunteers instead of underpaid employees.
-Joe Offer-