I love Yoga One. I came back to yoga in the last few weeks and feel right at home. I love the instructors. The power flow classes are fantastic. It's called flow so there is almost constant movement. (This comment is in response to one of the reviews posted.)
Since coming back to power flow a few days a week I can tell the improvement I've made in my strength. I love that they've also added deep stretch classes. Since I'm training for an athletic event, these classes have been a phenomenal addition to my training. The class helps reduce my soreness and release any tightness. Each class offers different focus areas (all of them have some form of hip flexor work).
If you can attend an assisted class I would do so. The assistants help with your poses and really help push your poses a little further.
What I also appreciate about Yoga One is there commitment to the community. The Karma classes generally support some community cause.