From what I have seen and heard over the past, straight bar curls (including reverse curls) have built more pure muscle mass as the number one bicep exercise. Either with a Olympic or just a plain exercise bar. People tell me there always seems to be something lacking when doing curls with the EZ bar, just by the feel of the exercise. Heavy, slight cheat, straight bar curls seem to be the way to go for them.
But if you put you empty hands in front of you, like you were holding a bar to curl, the hands will be in a natural EZ bar grip, turned in a bit. If EZ curls work for you the best, with less stress on the wrist, than go for it.
EZ bars work well for upright rows and triceps for a lot of people.