I have an Intellimouse Explorer (5 button mouse). I've installed the Microsoft drivers on OS X (Lion), and the extra buttons work fine with the mouse directly connected. But when I share the mouse from a Win XP Synergy Server, the mouse buttons stop working properly - buttons 4 and 5 seem to duplicate button 3.
So, I guess the problem could be:
Any ideas how to get this to work? I can't manage without back/forward in the browser :)