What is it you're skeptical about? This is not for everyone, but then again neither is yoga, Pilates, step aerobics, rock climbing, marathon running, or any other form of exercise you name. And why would it get boring? Again, I could say the same thing about any other form of exercise you name. What could be more boring than walking on a treadmill?
Believe me, this is only one part of my regimen. There are three levels for the barre -- beginner, intermediate, and advanced. It also uses a workout ball and resistance bands. No, it's not cardio. Neither is my Bowflex or my free weights. What this is is using your own body weight to strengthen and stretch your muscles, and not just your legs and abs. One exercise in the beginner workout (the only one I've done or viewed so far) is to lay flat on the floor, keep your body in a straight line, and then do pull-ups. This works your core, your arms, your chest, your shoulders, your upper back.
I also do my triathlon (mentioned above) for cardio, and my Bowflex and free weights for weight training.
I'm not sure what you mean by "no progression." You get stronger and leaner. The ab exercises are harder than any I've tried before. There's no progression in terms of added weight, but there certainly is progression in the sense that you do harder and harder moves, which burns calories and fat and builds muscle. After my first workout, I was definitely sweating.
And, this will definitely help with my flexibility.
As for the supplements, I'm now taking one high-potency multi AM and PM, 1000 mg Borage Oil (gamma linoleic acid) AM and PM, 198 mg potassium AM and PM, 142 mg magnesium malate AM and PM, 1000 mg Acetyl L-Carnitine AM and PM, and 300 mg Alpha Lipoic Acid AM and PM. I vary with adding other supps from time to time, but this is my base.