Mouse buttons 7 & 8 missing.
Undaunted, I installed the two other competitor drivers out there, [SteerMouse|http://plentycom.jp/en/steermouse> and [ControllerMate|http://www.orderedbytes.com/controllermate>... both of which were also unable to receive any mouse events from my missing buttons.
It is my belief that Apple added "support" for mice like this in 10.5.2 and such support is found somewhere in the kernel, but I can't stress how much this is a bad thing. My MX518 is now a 6 button mouse, and whereas I used to use every button routinely in programs from Photoshop to World of Warcraft, that functionality is now missing.
Help. (or, Apple if you're reading this, please fix it)
MBP 2.4GHz, Mac OS X (10.5.2)