I've been a vegetarian for years, but an overweight one. I switched to a lowfat vegan diet (McDougall) January first and have seen lots of improvement. I've lost 20 lbs and feel really good (haven't been sick, no bothersome allergies, etc) even despite a few setbacks and several days where I "cheated" and ate unhealthy junk food or even a few dairy products. The one thing I find to be wonderful, is I didn't get a sinus infection during the few nasty months of winter, which used to be a reliable nasty thing. I attribute this to cutting out dairy (well, minus my few slipups).
I still have quite a bit of weigh to lose (I'm currently on a 5 day fast to jump-start that and get myself over this little plateau I've reached) but it is nice to feel like I'm almost half way there :)
I should mention too, that I plan on continuing to eat McDougall-style (www.drmcdougall.com) after my fast is over with.