Vash, I always enjoyed yoga and am considering taking classes again. I never got past the beginner classes when I went, but the yoga poses were great for strength building and stress relief. I also got nice, toned muscles without having to do crunches.
Also if you have bad knees (like me), some teachers recommend putting a towel if you're going to put weight on them. I even have to put a towel down to do those cat/cow stretches.
From what I know, you're never supposed to do a pose so intensely that it causes pain. Be careful. I have a friend who got a hernia from an instructor who pushed her into a position (not sure which one it was), and my friend didn't complain about the pain it was causing her. This friend also had an eating disorder which I'm sure contributed. But even so, I would always say something to the teacher if you're in pain or discomfort since usually there are modified versions of the pose you can do which won't hurt you and will still give you the benefit.
“Inside every cynical person, there is a disappointed idealist”--George Carlin