Well, I exercised totally at home for 2 years, while I was in grad school, except for one summer of yoga classes at a studio. I have a trainer, a treadmill, some weights, a ball, yoga dvds, and a winter cycling training program, that has lots of fucntional fitness/weights stuff. It was OK, I didn't gain any more weight or lose fitness than usual, but I did start to miss the group peer pressure. Mind you, I get most of my exercise outside, through riding, hiking, winter stuff. So, I joined a cheaper gym and go mostly for the "boot camp" classes. Occasionally, I go to a spin class or a yoga class. I think going to these boot camp classes 2 days a week has made me a lot stronger, and introduced me to new kinds of stuff. Some I could (and do) do at home. If you can find a DVD with these types of exercises, it might help, if you want to do it at home.
I am pretty disciplined, but it is much easier to cheat doing it at home. I allow myself to give in a lot more when I am riding, than at the gym! I need to get back to doing more group rides.