It's a really great opportunity to get up and learn to teach your own class rather than just working with a professor as a TA. Really comprehensive and rewarding experience.
Cons
The pay is unlivable unless you're not paying rent. The contract is 15 hours a week, but obviously if you're just starting to teach as a GA, you put in WAY more than 15 hours of work. If you come up against a problem, there is very little oversight or support.