I've been vegan for... I think 17?... years. And very healthy overall, but by no means thin the whole time. During the time I've been vegan, I've had weight fluctuations and tend toward slightly overweight if I don't watch what I eat and exercise regularly. Lately, let's be honest, I've edged past "slightly" and am on here to get back on track and hopefully lose 30 pounds or so. Who knows, it's possible it'd be 40-50 if I wasn't vegan, that's hard to predict. But there are lots of high-calorie vegan foods and even healthy foods will cause weight gain in large portions.
I would say that there are great health and ethical reasons to be vegan or mostly vegan, but not to treat it as a fad diet where you supposedly eat as much as you want in certain categories and watch the pounds fall off without ever thinking about it or feeling hungry. It doesn't work that way. Of course, if you use the tools on this site, you'll be off to a great start in learning what a healthy, nutritionally complete vegan diet looks like and how many calories that Thai tofu & vegetable curry will actually rack up.
I've never looked back and never really missed most animal products (and there are so many substitutions these days). If you live and spend most of your time in places and with people that will accommodate your dietary choices, you'll be fine. Most restaurants have something vegan (or that can easily be made vegan), and you can always bring something vegan to social gatherings. Traveling in some places can be a challenge.