http://southsidesoaring.com/South_Side_Soaring/Home.html
I am a member... a small, active club with a decent towplane (150-hp Citabria) and two good gliders (a 2-33 and a 1-26). Three seasons for me now; got my ("add-on") PP-G during the second. It is challenging, inexpensive, social and team-oriented, and yes, very good for the most basic pilot skills (including the way you think about everything). I will return to power flying one of these days, but I'm definitely hooked on soaring.
The club's not really an "on demand" type of operation... calendar time needed to finish the PP-G may vary. You join the club with a deposit that buys you a share of the gliders, pay nominal dues, pay nominal tow fees, and instruction is free. We have two excellent instructors, both also power pilots... one of them a very seasoned glider instructor, the other a new glider CFI but a long-term CFII-rated power instructor. Part of the reason it took me more than one season was the fact that we only had one instructor, who works and has family obligations, but the new guy is retired and is available more often.
Our typical flying season is April to November, and we fly at least once a week (usually Sunday). But it can be arranged to get a tow just about any time. But on a long summer Sunday, even with other students flying the trainer, you can bang out a few short hops, and move along pretty quickly.
Right now we only have two members working on their tickets, out of about 8 actively flying members. Most of the other members own an interesting variety of power planes, and they are pretty good about sharing them.