I'm currently using a Rival 300, the only extra buttons are on the left side of the mouse and I mouse with my left hand so I don't even use them. (All of my abilities are bound to the numpad side of the keyboard.) I've never really found a need for a MMO mouse (especially since I'm a lefty) even when I played a Shaman back in TBC/early WotLK and had to drop all of my totems individually.