Good motivator, until you lose your progress.
But the worst has to be the stability of the game. I was 4 days from the end of a 30-day program, when my save file was wiped out -- all of my history including the program and personal data, so I had to retake the fitness test just to get back in. Yesterday, the game restarted in the middle of a workout. I was able to skip the exercises I had already completed, but I didn't get credit for them although I got an "A" on them.
When I first got the game, I was stoked. I waited patiently to recover from a cold so I could jump in at full strength, and I almost made it a full month with a daily workout (with occasional days off, per the 30-day program schedule). But having the game choke on me twice, including losing all of my data to that point, has made it harder for me to regain my pace.
If I had a live-human personal trainer who performed as well as this game does (not), I'd have fired him by now.