This is a question many more people are asking these days especially in light of Mad Cow Disease and the more recent outbreak of Hoof and Mouth Disease in Europe. The answer, unfortunately, is 'it depends'. Firstly, a vegetarian lifestyle is not just about eating vegetables in place of meats. It is important to know which foods belong to which food group, which contain necessary nutrients and which are deficient. BUT they are also often high in fat as well.
Many vegetarians suffer from low iron levels and some are deficient in some of the B vitamins.
Portion size also is important. You may have heard that rice is a complete protein, it is balanced, and that in some countries people live well on rice. However, in North America, we have a tendency to eat white rice, instead of the more healthy brown or wild rice varieties, and this leads us to consume more starch or carbohydrates without the same amount of protein to balance it. The same could be said for a reliance on whole wheat pastas or breads, and cereal grains. All the food that we take into our bodies is broken down into sugars for use in the cells.