I don't know much about them, however hopefully this can help.
--Logitech MX518 Gaming Mouse
We fired up several games to check out performance, including Unreal Tournament 2004, Novalogic's Joint Operations, and Microsoft's Rise of Nations. Switching to a lower DPI proved to be quite useful when zoomed in with the sniper rifle in either of the first person games. Rise of Nations seemed to benefit less, although it was useful to "downshift" a bit when we had to zoom in close to micromanage a battle. Unreal Tournament's sniper rifle has a highly variable zoom. The effect of shifting the DPI was greater when zoomed in to the max.
Users with short, stubby fingers will have to reach a bit harder to press the (+) button.
Frequent shifting in a game could tire the forefinger.
The MX518 ships with three built-in DPI toggles. Defaults to 1600DPI. Pressing the (-) button once downshifts to 800DPI; pressing a second time gets you to 400DPI. The cool thing about this is that you do not need to have the driver software installed.
Logitech has built very slick (literally) feet on the bottom surface of the mouse, to enable smoother glide as you're moving the mouse around. While this allows for faster mouse movement, it also makes mouse movement easier.
--Logitech MX610 Gaming Mouse
Comfortable; precise; excellent tilt wheel; volume controls; long battery life; long range.
New design looks more stylish and professional than the bean design, and is just as comfortable to use. Picking up the mouse is without fault, never slipping and for the most part balanced with a little more weight in the rear thanks to two AA batteries. The only real difference between the two form factors is that the MX610 doesn't have the centimeter of space for the ring finger. With only the edge to rest upon, we found my ring finger unable to decide if it wants to sit on the right mouse button or join my pinky finger on the side.
Recap
In the end we were left with a mouse that fell short of expectations. Cheap, comfortable and precise, the MX610 has the markings of a great mouse. The innovation of two way communication allows for excellent battery life - after nearly a month of testing, the two AA's are still "good". However, the lack of compatibility with AIM and Thunderbird hinder the MX610's usability as a pager, and depending on your setup you may encounter unsightly wireless interference. If possible, try before you buy.
IMO, Cade, buy the MX518-- It by far surpases the MX610