While I recommend AFS, I do so with reservations, as I would for any study abroad program. Did the year I spent in Germany in 1981-82 change my life? Absolutely, and I would never trade the experience despite all the problems I experienced, some of which were of my own making.
First: If you go for an entire year, do not expect to be treated the way foreign exchange students are treated here in the US. At my high school in California, foreign exchange students were put on the homecoming court, encouraged to try out for sports teams, allowed to go through graduation, and generally were given as many opportunities to experience being a teenager in America. When we took in a foreign exchange student for the summer when I was young, we took them on vacation to the Grand Canyon, Yosemite, San Francisco, and the California coast. We took them to Disneyland and Hollywood, San Diego and (only because they wanted to see it) Death Valley. In Germany in 1981, the school could care less that I was there and many of the students saw me as a stereotypical American - as if I represented Ronald Reagan himself (Germans saw Reagan's rhetoric as dangerous). Even my German parents were anti-American to a large extent. No matter how hard I tried to immerse myself into German culture and the language, I was rarely accepted as a young 17 year-old that just wanted to learn about a new culture and improve my German. Of course, American and British troops ran war exercises nearby running tanks through the streets, and American nuclear missiles dotted the countryside, so looking back you can understand what made many Germans upset. As a young America, I was clueless to all of this. The breaking point came one night when my German parents found out I was a Christian Scientist and then proceeded to trash my religion and my parents for raising me to believe in it (they were Lutheran, but didn't go to church, not unlike many Germans). To their credit, the local AFS people found another family that were much more caring and understanding as their own daughter was in the US with AFS in Ohio, getting all the experiences that expected when I went to Germany. While this was an awful experience, I learned to expect that people often see the worst in you, often based only on your nationality. I know this applies to Americans as well, I had just never experienced it as a child as my parents had raised me to be accepting and open to everyone. Still, I must say that at that time Germans were mistrustful of Americans and foreigners in general. They were much more willing to send their students to the US (over 250) than they were to take in Americans and other foreigners (only 80). This may be very different today.
Second: Rememeber, it's a study abroad program. You live with a family and go to school for a year in a completely different school and social setting. I say this because a few students that had to go home during the year failed to see this point. They expected the kind of freedom that most American teenagers get in the US, including choosing your own classes or being allowed to go out at night with friends; this was not how things worked in Germany. If you want to party in Europe, go on a vacation.
Like many students, I had graduated from high school so taking a full slate of classes was a shock because my German was not very good when I first arrived. I asked the school if I could take an intermediate German class instead of Latin, but the school and my parents said no. I found out later it was because the program was mainly for Turkish immigrants, and my German parents openly hated the Turks (my second
Third: It is expensive, so be aware of what you are spending your money on. AFS isn't perfect, but some of the negative comments suggest that parents or their children expect everyone to bend to their idea of what the experience should be. AFS cannot guarantee the perfect experience. In hindsight I probably would have had a much better experience going abroad in college when I had a few more years of life-experience and the ability to travel on my own or with friends on holidays. I was not mature enough to understand this at the time. And if you sign a contract, read the fine print!
In short, I would only recommend you to do a foreign exchange program if you are mature for your age (I was not), almost fluent in the language (I only had 2 years of German, not enough), and have not led a sheltered life. If not, wait until college.