A great program, but your experience is 50% luck, 50% your attitude.

Pros: Teaching in Spain for 8-9 months with health insurance is a pretty good deal! You will likely have a built-in support system from the school and teachers you are working with. You'll also have a good chance to work on your Spanish, as the students often need you to translate for them. You'll be able to save up money while living abroad, making it easy to travel to other places in Europe. You can also scrape together more cash by teaching private lessons.

Cons: The quality of your experience can vary greatly depending on the region you are placed in, as each region in Spain is responsible for payment of its own language assistants. (Some regions, like Castilla y Leon, are better about paying assistants on time; Andalucia and Catalonia notoriously pay their assistants late.) Also, you're obviously going to have a different experience if you are placed in a small village versus a city, like Madrid. It's a bit of a coin toss, considering you are not guaranteed to be placed where you'd like.

Also, the application and visa process can be daunting at times, and often the program organizers simply don't have the answers you'd like them to.

Final word: Overall, a great program. The benefits are obvious, but be aware that there are downsides. I wouldn't recommend this program unless you are very comfortable speaking Spanish (or are participating with someone who is)...or unless you are a seasoned traveler/expatriate. I don't think I would recommend program for a pure anglophone's first time living abroad.

P.S. If all you have is your high school Spanish, take some classes before you go! You'll thank yourself later.