Considering the amount of knee-straining poses my teacher makes me do, yoga is not necessarily easy on any given joint. Overall, it's much easier if you'd define your goal first and then pick the exercises you're going to do based on what will actually get you closer to your goal - yoga only really works for an extremely narrow goal space, which is why my yoga teacher lifts weights with me because his yoga practice can't do everything he wants.
I certainly understand the knee-straining poses quandary. I believe it's more 'straining' if you remain in the pose for an extended period. That's what I like about the DDPYoga program; limited time in any one pose.
I've found it to be very effective myself.
Dusting this thread off as I just started DDPYoga myself.
Anyone out there that still is doing it or just started?