I have been vegan for 4 years now (there are others on here that have been longer) and it has been great! I have never really followed a plan, but took resources and recipes from many different places. I have never felt better in my life. The key is eat a varied diet and not eat the same things all of the time. That way you can get the nutrition that you need. The only real thing you should worry about is B12 and possibly Vitamin D. You can get these added to your normal blood work to make sure everything is in check and take supplements if necessary. I have more energy than anyone I know and neither me or my husband (who has been vegan just as long) are pale or grey-skinned (common vegan myth!) or have had any adverse health problems from being vegan.
I feel great as a vegan, and won't return to the way I used to eat, but definitely will keep my B12 level medically stable. I wrote about it recently on my blog. See "Some Notes on Labwork" in archives.