CAP
I was a CAP member for six years. Some of my fondest memories and best flight and personal experiences were from being in CAP. I got some early flight instructing experience in CAP, which was valuable, but the best experiences I had were from the fellowship and comaraderie, and the learning from the personal development and aviation-oriented courses and seminars. Note that I put aviation at the end, not because those experiences were of low quality; they most definitely were not, but because the personal development courses and training were so meaningful.
The quality of CAP experience will depend very heavily on your unit. I was extremely fortunate. Most of my CAP experience was in Oklahoma Wing. My unit was in Oklahoma City, and we had a great deal of contact with Wing staff. Our wing was run very professionally, and that greatly influenced my CAP experience. I got to know a few members of Wing staff. They mentored me and presented me with many opportunities to develop and hone my personal, organizational and leadership abilities. Our Wing flight standards officer was a real, live fed from the Monroney Center. His day job was to train DEs. He developed a course based on his DE course for wing and squadron check pilots. I took the course and helped put it on in my capacity as an Assistant Wing Check Pilot. That training stood me in good stead after I went to work at ERAU. I was promoted to Stage Check Pilot at Riddle. I had known for some time how to give a flight test per the FAA, so I was ready.
I saw the difference when I moved to Arizona. I joined the squadron near ERAU. The squadron was not run professionally at all. I went to one practice SAR and was disappointed with the lack of organization. I went to a wing conference and was similarly disappointed.
Once again, a lot depends on your unit. There may well be bickering, politics and agenda-promoting. You see these things in volunteer organizations. By the way, because CAP is a volunteer organization CFIs cannot charge for their services if performed in CAP airplanes. But, take it from me, in the right situation there are other rewards.
http://www.cap.af.mil may be the website. It didn't work when I tried it.