Felt like chiming in here with my story. Sometime last year some co-workers of mine were going to start P90X so I decided to join in. When I started I just had the bands and a mat, which were fine but the bands eventually broke because they weren't actually fine - instead, they were cheap pieces of crap. In any case, I made it through the first 2 months and then went to the hospital because of either an appendix issue or food poisoning (no one was sure).
I restarted from the beginning because too much time had passed, but this time I added the Bowflex Selecttech weights to the mix instead of bands, and some inexpensive push-up bars. Unfortunately, in the 3rd month of this round - I had maybe 3 weeks to go - I quit again because I did something to my right shoulder. Not sure what, but sometimes it hurts to raise my arms above my head. Also sometimes hurts when I do wide fly pushups. Ugh.
Anyway, I took a few weeks off but the best thing about this was that all this quitting made me realize how effective the program actually was. During the first two rounds, even though I didn't finish, I lost a shitload of fat and put on some visible muscle - especially in the chest/biceps/triceps area. I was getting a lot of comments from friends about how thin I looked - and I've never really been thin since I was 10 (never fat, either, just chubby, I'm short and stocky like Russell from Survivor). My average was about 190 during college and I'm 5'7". So I had some fat to lose. During the program, I was down to mid-170s but that was definitely muscle weight with a decent amount of fat lost.
During the downtime after the failed 2nd round, I just felt fatter. My face felt fat. I could see and feel my arms, chest, and stomach losing muscle mass and replacing it with fat. So I just started round 3 with gusto. I'm going to try to do it a lot smarter this time, and thanks to the previous 2 almost-done rounds worth of experience I should be able to give it my best shot yet.
Anyway, what I'm saying is if you give up once or twice it's no big deal. Just keep trying. Depending on how far you make it before you give up you should start to notice the effects wearing off and realize "holy shit, this really was working" which may help you start again.
One question - what do you people use for a recovery drink (if you do)? I was originally using some Clif Shot recovery mix but damned if it didn't give me some insane heartburn every time. I switched recently to some Muscle Milk, which is a protein drink instead of a straight up recovery one, but it tastes pretty good.
