There have been quite a few well known individuals who have decided, after many years, to stop being Vegan.
Remember the movie "Supersize Me" by Morgan Spurlock? His girlfriend Alexandra Jamieson was a devout vegan who was upset with his McDonald's experiment. She is no longer Vegan.
Read about her change here.
Also, Kristen Suzanne, A Raw Vegan Chef and Author is no longer Raw or Vegan. You can read her story her here.
There are a slew of others as well which you can find if you search around a little bit online.
I was a Vegetarian for a very long time. I had two bouts of horrid food poisoning when I was younger and just became grossed out at eating meat and poultry. I dabbled back and forth for years between being Vegetarian and Vegan from about age 12 until I became pregnant at the age of 26.
I am a total and utter meat eater now. Obviously, this is my own experience in this matter but, as a Veggie, I was doughy and tired. I always felt like my body was missing something but I just couldn't put my finger on it. I had digestion issues from the types of Vegetarian and Vegan foods I was consuming and had no real muscle tone or strength in my body. I ate meat and poultry through out my first pregnancy and once I had my son, combined with a regular fitness program, I got right back to my pre-pregnancy size within several months.
Honestly, in my particular circumstance, I have a hell of a lot more muscle now than I ever did as a Veggie. I ran an Ultra! Talk about energy! I'm not trying to preach to anyone about what's best for you, that's something everyone has to figure out on their own but many of the Vegetarian and Vegan products contain Soy and I am convinced that Soy causes cancer and a host of other nasty problems.
I believe in eating foods as close to nature as possible. I don't like additives and preservatives and strange chemicals. If I eat chicken, the only ingredient is going to be chicken. If I eat broccoli, the only ingredient is going to be broccoli. I feel stronger than ever as a meat eater and will continue to pursue my nutrition in that way.