Junior level stuff is really worth it, from what I've heard from friends. I only got into tennis late in life, and it is really hard to improve without lessons, which cost a lot of money. The fundamentals are really important in tennis, and junior level stuff tends to teach it pretty well.
Being involved in school or junior usta tennis is great for many reasons, but for me the most important would be that it's much easier to get proper instruction for little or nothing. The other pluses are the fact that most people will be about your same age, making it social, and also the fact that people seem to take it not too serious but serious enough.
Adult leagues are a mixed bag. Some people play too competitively and some not enough. Everyone's a different age and you don't really form a friendship with them.