Forget Logitech. Five buttons is where it's at, and Logitech fails to deliver this.
Two great choices.
1. kärna Precision LLC: Razer Boomslang 2000 dpi.
2. Microsoft IntelliMouse Explorer 3.0.
The latter comes with IntelliPoint drivers for Mac OS X. The first does not, so you need USB Overdrive, for instance, for full functionality.
The MS one isn't for lefties, by the way. The Boomslang is symmetrical so lefties aren't left out (no pun intended).
I hear Kengsingtons are okay, too, but I haven't tried one, so...
Oh, and, be sure to install MaxMenus and have one of your mouse buttons pop up an app/shortcut menu. A really good way to improve your workflow.