Thanks to Bijan for the referral. David, I own and operate Arizona Soaring inc near Phoenix. We are, as Bijan mentioned, the largest glider training school in the US. About 2/3 of our clients are doing some sort of "add-on" rating(Pvt, Com or CFIG). We are just one of several quality schools around the country plus there are some clubs that also do a good job of training. Many of our clients are club members from around the country and choose to train with us. Some advantages to working with a professional school such as ourselves are:
1. Being able to fly as much as you want.
2. Escape to a nicer climate during the winter months.
3. Availability of FAA Examiners, instructors and aircraft.
4. Being able to learn from instructors that teach in gliders every day.
As with choosing any school, it is important to find a school that has a good reputation, safety record and availability that fits your schedule.
Glider flying is a blast. I'm certain that you will enjoy the training process, learn some great skills, and meet some wonderful people too.