I have the same experience as 21277. I'm also an A blood type and tried being a vegetarian (even a fruitarian for several months), which made me gain weight and gave me high blood pressure. So I don't believe in the blood type diet. I do believe in metabolic typing though. I'm a fast oxidizer, which means that I do better on a diet that's high in protein and fats, while lower in carbs.
However, if you're a carb type/slow oxidizer, then a vegetarian/vegan diet may work for you.