In general yoga is always great. The quality really depends on the teacher however. I had a teacher of Iyengar style who was somewhat strict in his approach. His heart was in the right place but he was too focused on "correct" posture.

After a good yoga course of ~1h30.. your mind should feel quiet. If it isn't then it's not a good course. Over time, you should also become aware of bodily tneisons in various areas. ANd perhaps even become aware of old memories, perhaps even cry. Those are completely normal aspects of yoga when it has a therapeutic potential. It happens as consciousness is more evenly distributed in all parts of the body. It restores the left/right brain hemisphere balance, it takes you out of the map, and more in touch with reality in its raw form.