99% Vegan here! (I do occasionally succumb to pizza with cheese
I do believe that everybody has different needs - we just need to listen to our body and the vegan diet is not for everybody!
as for B12 - one can get enough through dairy and eggs.
as for getting sick - for me, I have been the same as when i was eating meat.
My husband has been vegan for 11 years and for the 5 years that i know him he has never been sick - not even once!
here are some stuff from wikipedia:
Health benefits and concerns
Vegetarianism is considered a healthy, viable diet. The American Dietetic Association and the Dietitians of Canada have found a properly-planned vegetarian diet to satisfy the nutritional needs for all stages of life, and large-scale studies have shown vegetarianism to significantly lower risks of cancer, ischaemic heart disease, and other fatal diseases.[10][11][35] Necessary nutrients, proteins, and amino acids for the body's sustenance can be found in green leafy vegetables, grains, nuts, and fortified juices or soymilk.[36]
Vegetarian diets can aid in keeping body weight under control[37][38] and substantially reduce risks of heart disease and osteoporosis.[39][40][39][40] Non-lean red meat, in particular, has been found to be directly associated with dramatically increased risk of cancers of the lung, oesophagus, liver, and colon.[13][41] Other studies have shown that there were no significant differences between vegetarians and nonvegetarians in mortality from cerebrovascular disease, stomach cancer, colorectal cancer, breast cancer, or prostate cancer.[42]
The American Dietetic Association and Dietitians of Canada have stated: "Vegetarian diets offer a number of nutritional benefits, including lower levels of saturated fat, cholesterol, and animal protein as well as higher levels of carbohydrates, fiber, magnesium, potassium, folate, and antioxidants such as vitamins C and E and phytochemicals."[35]
Vitamin B12
Plants are not generally significant sources of Vitamin B12,[51] However, vegetarians can obtain B12 from dairy products, eggs, fortified foods and dietary supplements.[52][53] Since the human body preserves B12 and reuses it without destroying the substance, clinical evidence of B12 deficiency is uncommon.[54][55] The body can preserve stores of the vitamin for up to 30 years without needing its supplies to be replenished.[51]
The recommendation of taking supplements has been challenged by studies indicating that exogenous B12 may actually interfere with the proper absorption of this vitamin in its natural form.[56] The research on vitamin B12 sources has increased in the latest years[57] and researchers at Hiroshima University have developed methods for growing plants rich in vitamin B12.[58]