+1 to Christcorp.
That said, I'm a cadet 1st Lt. in CAP. I've been involved 4 years and it's been extremely valuable.
If you're considering joining, you should definitely go poke around on the website. Also look for a unit in your area. If you're lucky, there might be several within driving distance. I'd visit all the ones that are--different squadrons have very different cultures.
Basically, CAP is about Emergency Services, Aerospace, and Cadet Programs. Emergency Services is the least self-explanatory... in my area, it's a lot of search & rescue training. Another significant component is disaster relief--things like Hurricane Sandy.
Aerospace is just about what you'd expect. Some squadrons are composed entirely of "airplane geeks." I personally haven't taken flight lessons, but perhaps 5 cadets in my unit are currently working toward their pilot's licenses, and another got hers this summer. It's a great opportunity for them because the CFA is donating his time... all they have to pay for is gas and the (very reduced) airplane cost! Scholarships are also available, and in many squadrons there is a heavy aerospace knowledge component (tests are a requirement for promotions, so that makes sense.)
The last and--to me, at least--most valuable part is Cadet Programs. This is integrated into the rest of the program to differing extents in different squadrons. In mine, cadets run everything. We have some senior member oversight, but mostly they let us do the work. In other squadrons--especially those with lots of young cadets, or with very few cadets--senior members are more involved with planning, instruction, etc. As well as learning formally about leadership, you get to apply it. I've already seen benefits in applying skills I've learned (administrative, motivation, etc) to being a good team captain in other activities.
CAP has NOT been a rosy experience every day, but it has definitely been worth it. If you are in middle school or are just starting high school, I would highly recommend checking it out. If you're a little bit older, you may still want to look at it, but bear in mind that depending on the unit you may not have enough time to get a leadership role. I'd plan to try to spend at least 2 years active (attending nearly every meeting) to get the most out of it.
Also, a caveat--from what our alumni cadets say, CAP will not prepare you for most aspects of the military. Much as most cadets would like to pretend otherwise, we are a civilian organization as well as the AF auxiliary. Don't expect to get a complete picture of "what the AF would be like." Maybe a taste. Maybe. But if you join, join for its other merits.
EDIT: As HeWantsTheBFE pointed out, there are many, many great opportunities at a very reasonable price once you've put in some time. Go to Encampment and then check out NCSAs! COS is far the best one I've been to, but I've also heard great things about IACE, Blue Beret, etc.
Many cadets in my squadron have gone to leadership, engineering and other flight academies.