My wife is a vegan, and I eat meat (although alot less than I used to).
We both feel much better and have more energy. It has no effect on our weight at all. If you want to be a vegan, it's important to eat correctly. So many of the PETA knuckleheads don't do it right. If you eat stupid, it will effect you - regardless if you eat meat or not.
Sometimes it so hard to have a science background in the US, because you find yourself surrounded by people who don't understand the world around them.