I have my own treadmill, and I'm very happy to have it. I DO find that it works best as part of an exercise program that includes other things.
What I like about having my own treadmill:
1) I had to start out very slowly - five minutes, and I would have been too embarrassed to do that at a gym.
2) It's good for inclement weather.
3) I can wear the same sweaty clothes more than one day.
4) I can control the difficulty of the workout.
5) There's no "extra" time, for example, driving time to a gym.
On the down side, I find walking outside to be much more of a stress reliever. I look forward to walking outside, and I come home feeling happy after a nice walk. When I'm on a treadmill, I'm just happy the workout's over.
If you're ever interested in buying a treadmill, I highly recommend this site:
http://www.treadmilldoctor.com/
They review exercise equipment. I've bought two treadmills in my life, and I sure wish I'd read the reviews before I bought the first one.