I recieved this mouse for free, near-new from a friend, as I did my Intellimouse Explorer 3.0a before that, so I think I can speak without the bias that comes along with wanting to justify/defend a given purchase. I think the MX510 has superior ergonomics (if you're right-handed, natch), particularly with the two side buttons located above the thumb rest. They are on a high ridge, compared to the Intellimouse, and are therefore much more difficult to press acidentally while surfing or playing games. The left- and right- click actions have their buttons seamlessly integrated into the top of the mouse, which is one less thing that can break in the long run. The scroll buttons on the top and bottom of the scroll wheel are nicely placed and useful, even for people with relatively small hands like myself. The MX510 also has some decent heft to it, considering that it doesn't have a tracking ball or batteries inside it. However, the mouse wheel itself is much stiffer than what I'm used to with most of the other mice I've used.
I don't recommend installing the software, however, as I've heard it's a bit glitchy and the pointer acceleration isn't very useful. Without it, you'll get still get functionality for all the but the task switching button. I never use that anyway, and it's also awkwardly placed. All in all, the MX510 tracks excellently and precisely, it just needs a smoother scroll wheel.