Countless people start P90x or Insanity or 10 Minute Trainer or any program and never finish. They aren't seeing results like they saw on TV so they just give up. Remember, this isn't a 3 month investment, it's a lifetime. You didn't get out of shape in 6 weeks. Why would you expect to get in the best shape of your life in 4 weeks?
If you want the infomercial style gains in 60 or 90 days, you need to have 100% commitment from day 1 on both the workouts and nutrition and probably be blessed with some pretty good genes. If you want the best results you can get in 60 days or 90 days, you need to have 100% commitment to nutrition and the workouts. Any less than 100%, you'll be on your way, but the progress will come slower.