Bought the MX-610 today and am testing it out. Haven't installed the software. Feels a lot different from my other meeces which are (very) old MS Intellimouse Explorer 1.0A. I've stuck with them because they fit my fat pudgy hands very well. Also, the scroll wheel is smooth compared to other incarnations of the Intellimouse Explorer. I had an IE 3.0 mouse and gave it to my mom because I didn't care for how the scroll wheel ratcheted when using it. This new mouse's wheel seems pretty smooth but not as smooth as the Logitech Revolution, which is the smoothest scrolling wheel I know of.
My only concern with this new MX-610 is if I can use two of them on two different computers that are sitting next to each other as they may be set on the same frequency.