Most of the replies to this thread are missing the point. He already HAS a 5 button mouse. What he wants is the ability to USE those buttons.
Left-click, right-click, and scroll are all handled natively by OS X the way you expect them to work. This means that any program written to Aqua guidlines will utilize those features.
OS X also supports many more than 5 buttons, but at this point programs must be written to take advantage of it. i.e. Return to Castle Wolfenstein, you can assign anything to all 5 buttons.
What OS X does not have yet is an equivalent to the MS Intellimouse software, which allows you to map key combinations and sequences of commands to a mouse button. I know that in OS 9, you could use this software to outline a button set for individual programs, which was a great feature. In IE, the thumb buttons were Back + Forward. In Photoshop, they switched to common tools(magnifier and select).
The uses for such software is really limitless, I don't understand why everyone is ragging on the use and advantage of multiple buttons.
FaRuvius