I'm a few days in, and my feet are pretty sore. I've been trying to get into it slowly, with sitting breaks. What has surprised me the most is that I don't feel more drained at the end of the day - if anything, I have more energy than before.

edit: one negative I have noticed - which I hope goes away - is with concentration. I'm having a hard time switching between tasks. Once i'm on one, I'm fine, but for some reason my brain is resisting starting new tasks more than when I was sitting.