Another anecdote: I went completely vegan ~3 months ago. I don't really "feel better". It's always the first thing someone asks when I say I went vegan.. "Do you feel better?" - I feel basically the same.

Although my weight has been trending downward, slowly, which is great since that's happening without trying to limit the amount I eat at all. That's very different from when I wasn't vegan, my weight would always trend slowly upward unless I was making a deliberate effort to cut back on calories.

BUT: I think the real reason I'm slowly losing weight on a vegan diet is the simple fact that eating a restricted diet (ANY restricted diet) requires you to think about and analyze absolutely everything that enters your mouth. It introduces a level of mindfulness that just wasn't there when eating an unrestricted diet. For me, this has had the side effect of also cutting out most processed food (read: junk food) from my diet, even though most junk food is vegan. I'm convinced this is the biggest value of being vegan.. it just makes you think about everything you buy at the grocery store, everything you order when out to eat, etc.

Another commenter calls this "paying attention to what you eat" - I think that's right on target, however cliche it may sound.