I've been vegan for a few years now. I started it one year by doing a 'trial week' in October/November sort of time to see how I'd get on, before I committed myself to doing Veganuary. I found it OK (I was a vegetarian before that so it was only about cutting out the dairy and egg products) and carried through December and January, thinking I'd go back at the start of February, and just stuck with it.
I found it easier to adjust than I feared. I didn't have any negative effects or feel tired or constantly hungry etc. I probably ate better because the restriction made me focus on what I was eating more and so I put more effort in to varying ingredients with the stuff I was making.
However I also never got these miracle benefits that you hear about. There's a bit of a cultist sect of vegans on social media who are like those Juice Plus people, who want to say 'I went vegan and I had sooo much more energy it was unreal, my skin used to be spotty and it all cleared up'. I felt a bit healthier but nothing ground breaking.
What I also noticed was how your system adjusts and when you haven't had dairy products for a while, your system isn't used to them. One time I was at a conference and they were serving coffees, they only had cows milk so I was like yeah ok I'll have it anyway. I hadn't had anything with milk for over a year at this point and I immediately found it really difficult to drink and got stomach cramps. That was pretty much my no turning back point that locked me in as vegan.
I'm not hardcore though and I don't tend to tell people I'm vegan unless it is out of necessity, because it tends to start a whole conversation about my dietary choices that I cba getting into a big discussion about. I won't buy any more leather clothes, but I still wear my old leather jackets because I figure its more environmentally damaging to throw stuff away and buy new, so I try to get the most wear out of everything as I can. Also I don't scrutinise the ingredients of everything I eat if I'm out, so sometimes some stuff I eat might have milk whey or whatever in. If it's in small enough amounts for my system not to get upset by it, I don't mind. If you are that sort of 'imperfect' vegan then some people like to latch on to it and call you a hypocrite.
A few years in to it, I'd say the pros are I eat more healthily and take more pride in cooking now than I did. I find it easy to stay slim and in good shape (although I run a lot anyway) and despite getting older, my running times did improve after I went vegan. Also my blood pressure - which used to always be on the borderline between normal and high right from being a teenager, fell a lot and is bang in the middle of normal range now. Is that directly because of being vegan, I don't know. Most likely, because being vegan made me pay more attention to eating healthily and my general lifestyle improved, and running more has boosted my aerobic fitness.
Cons are that I spend a bit more on food than I used to and I find travelling abroad a nuisance as depending on the country, a lot of places are not very vegan-friendly at all. But in the UK it's easy now.
Good luck to anyone trying Veganuary or even just experimenting with replacing some meat/dairy things every now and then. I find it more constructive to support others in doing what they can at their own pace rather than being a zealot about it.