The programming is the key to success in his program (Steady State- Training Cycle).
If you hang around his forum/board long enough you'll see other people's training journals and stuff. It's pretty easy to go through the first few progressions. If you already have the body type designed for bodyweight mastery (lighter types have an easier time with relative strength stuff), you really stick to the training and drill it like a skill you'll get efficient at it pretty fast. Rings are an EXCELLENT supplement to any training. If I had a qualified coach and a good gym I'd pretty much just olympic lift and do ring work (I'm unconcerned with weight gain).
The negatives... It can be fucking boring. Lifting weights is MUCH more gratifying. But if you can push your ego aside it's very applicable. Progress can be fast at times depending on the exercises, but when it's slow it's like 6 weeks to the next progression, and because most movements are largely skill based (when I say skill I mean more kinesthetic sense, you need acute awareness of your body position and just what you want to be activating to perform the movement...which are all called "skills" for a reason) it's often better to focus on very few skills at a time (like 1-3).
Another problem is that sometimes it's hard to measure progress imo (for the reason that progress is often very slow), but with proper programming you KNOW you're getting better it's just hard to show results till you end your cycle (again, just my opinion). For one arm chins for instance, I've had more success just doing weighted chins and practicing 1 arm negatives than any crazy variations.
The most immediate disadvantage is that it's like re-learning lifting all over again. I know how to raise my bench, I know what I need to work on in my squat and power clean, etc... But for the gymnastics work you're going to be doing a LOT of reading and research before you can start self programming for your weaknesses and such (fortunately a lot of it is just sticking with your progression longer).
The positives... The isometric nature of a lot of the progressions and the time under tension really lend itself to grappling where even when you're loose and relaxed, you may still be at 30% tension. You need very little equipment. Pair of rings and something high to tie them too, pullup bar is best. I left my rings at CSW lol. You can switch to a gpp/conditioning mode very quickly.
Pushups on rings are tough enough to be a good part of a metabolic circuit but not so easy that it's a minor annoyance. Once you're doing over 5 sets of 30-50 on rings if someone asks you to do regular pushups it's almost insulting. Another plus is that it doesn't take very long at all. 20 minute session should leave you WIPED and your hands shaking like insert bad back to the future joke reference here.
He is supposed to release a book on explosive strength and flexibility (I believe "Liquid steel" was the working title), but I'm not sure if those have been released yet or maybe he scrapped the ideas. But anyway there is a TON of bodyweight training information out there. Just check youtube sometimes you find some bullshit and sometimes you find some gems. I saw this kid develop a planche pushup by just doing pushups with his feet on the wall in socks so he could slide down slowly and do negatives. Would never have thought of that myself.
If I chose two moves they'd be something simple like 1 arm chins and dips on rings (bulgarian dips, etc). Or planche variations and levers. But again, check his forums he gives away so much free information it's actually bad business lol. I hope that doesn't come across as an insult... it is NOT... but (at least maybe a year or so ago) he was basically writing programs for guys. There's so much info out there especially on his site you can pretty much put a training program together from just scrapping articles n posts together.
Anyway good luck,
Coach Sommer's Site: