Funny you should ask this, I spent a couple of hours reading about this very topic last night. Let me preface by saying that I'm not vegetarian, although I don't currently include mammals in my diet. I've considered going ovo-lacto-vegetarian, and since I'm a research junkie, I got to doing some reading.
A few of the strongest points that I came away with were:
1.
You can't generalize. Veg*an diets can work for people, but that doesn't mean they will work the same for everybody.
2.
You can't generalize. The standard american diet (SAD) is probably the worst version or an omnivourous diet. You can't extrapolate the health effects of the SAD to ALL omnivorous diets.
3. Evolutionarily, humans are omnivores. But that is hunter-gatherer omnivores, not agriculturual omnivores.
4. People that follow special diets become emotionally attached to their diets/beliefs, and it's hard for them to discuss them objectively.
The most balanced discussion I found on the topic was at
Beyond Vegetarianism--Raw Food, Vegan, Fruitarian, Paleo Diets written by some long-term former vegetarians and a current vegan.