I've noticed these:
None of SteerMouse, Microsoft Mouse and USB Overdrive works with 64-bit kernel. Only SteerMouse works with 32-bit kernel. Needs to recompile their kernel extensions to 64-bit to work properly. Flip4Mac and Growl should work in both 32-bit and 64-bit kernels. Plugsuit doesn't work, in either 32-bit or 64-bit kernel.
Every other program I've tried works with both the 32-bit and 64-bit kernel.
I'm currently using 32-bit kernel because I want to use the back/forward buttons on my mouse. Any other way to do this than USBOverdrive/SteerMouse/Microsoft Mouse?