I've been mostly plant based for about 12 years now, although I do eat some pizza and meat occasionally (and hey, a milk shake now and then). So I'm not religious about the whole thing. I went from having meat and cheese pretty much every day to only having it rarely, though, and I'd probably say it's the best thing I've done in my entire life.
It took me literally days to feel better and have more energy both physically and mentally, and I'm now 43 and I don't think there's any chance I'd be doing so well without eating this way. My training buddies (not to mention my non-training buddies) my age have definitely aged in a worse way than I have, and I'm happy to say that I've inspired some people to start eating this way, or at least cut out a lot of the animal based products they were eating. Notably, my GP is one of them. I go there for a check up probably every 18 months on average, had my last one in November, and those check ups alone are encouragement enough to continue eating like this. And that's good, because to be honest, I'd love to eat more meat, cheese, and seafood... I'd be lying if I said I don't want that stuff on a regular basis. I seem to do better in all ways when I don't, though. And when I do have some, I enjoy it more than I did when I used to have it all the time. So there's that.
I should mention that I'm eating a large portion of my food raw, and I eat almost none of the retarded processed junk that I see line most supermarket shelves in a lot of countries (like the one I'm in right now, Indonesia - people here eat a LOT of trash). That really makes a tremendous difference. You could call yourself a vegan and eat junk food all day, and a lot of people seem to do that. I'd rather eat clean and include some meat in my diet than do that. But just going from mostly cooked to mostly raw makes an overnight difference in energy levels, and I think most people find that they need less sleep on raw food as well. I'm not sure why that is, but I experienced it quickly myself and have since heard the same thing from others.
So raw fruits and vegetables are roughly 60-70% of what I eat normally. When I up that a little bit (to, say, around 80% or more), I feel just ridiculously good, so good in fact that I'm starting to get convinced that this is the way we're supposed to eat. But then it's hard to live that way in today's society, obviously - at least if you want to have a social life or not drive your significant other completely mad (and my girlfriend is a "flexitarian" but even she gets bored quickly when I do the whole monkey style eating all day).
I do have my vices (whisky is my main one, without doubt), but I keep them in check these days, unlike when I was in my twenties. Health wise I'd probably be better off without them, but I'm not a fucking monk either, and it all seems to work out fine, so I'll keep drinking whisky and enjoying myself until the day I die. Ultimately, life is about more than being a total health freak anyway.