My WWS experience was actually rather negative. While I was connected with a great school in Arizona, and I had a great relationship with the kids at my school in Bulgaria I hadn't anticipated that they would not like each other. I assumed that kids like kids... But when they exchanged letters they exchanged facebook addresses and the Bulgarian children refused to contact the American children anymore because they were, "ugly." (Racism is rampant in Bulgaria, and the school I partnered with was highly Mexican-American.) I feel bad that I wasn't prepared to deal with that situation, and I was never able to bridge that gap between the students. I did, however, remain in contact with the Arizona school, emailing them and answering questions about Bulgaria for two years.
In the end I would say it can be a great experience, but be prepared for it to go sour as kids can be brutal to each other at times. If you prepare for that, then you will probably be able to handle it much better than I did.