There is 2mg of b6 in a Monster and you need > 100mg/day to be over the tolerable upper limit set forth by the Institute of Medicine, though the lowest case report I've seen of toxicity has been at 500mg/day or so.
In any event, I don't think there's really anything wrong with them. Sure, there are a lot of artifical sweeteners and chemicals in them and I'm sure some clean eating folks won't like that I don't really care.