Egotomb: Guess it depends how you hold it, I also have one of these and I hate the position of the "slower scroll speed button" because I'm always pressing it. I could disable it if I installed the full driver set but still for ME it's in a dumb place.
In a nutshell if your a gripper you'll be fine with this but if you relax your hand over the mouse you will hate it as I do.
Definitely depends how you hold it. I'm not a 'gripper' (if I understand what you mean there) and tend to use the mouse with a very relaxed hand, but I do know what you mean. When I first got the mouse, I tended to hit that button a lot as well, but now I'm so used to the mouse that it's not a concern any longer. I rarely ever hit it by mistake.
But I find the general feel, comfort, and weight of the MX518, as well as the placement of the 'previous page' and 'next page' buttons are perfect for the way I hold it.