Logitech MX 700
Take the MX 500, remove the cord, and there you have the MX 700. The ergonomics and buttons are strictly identical, so I'll not go through all that again. The only change is in the weight, which is much heavier due to a bigger electronic unit and especially to the two rechargeable Ni-MH batteries. Some folks like heavy mice, but for the others, it's a handicap. It's true that really fast games like Unreal Tournament can tire your hand after a while, depending on its shape.
