I have a mouse with more than the usual 3 clickables (in my case, a Roccat Kova). Originally I got it because I was looking for better gaming performance (higher DPI, polling rate, etc.), and it's symmetrically shaped (re: ambidexterity). However, for some reason (that I can't remember) I ended up mapping two of the extra buttons to Page Up and Page Down.
Now, I use these paging buttons as complements to the mouse wheel for scrolling, and I have to say that this is one of those "how did live without this before" things. I honestly feel like it's similar to the transition in technology from wheel-less mice to mice with mouse wheels. Nowadays, it's a given that mice have scroll wheels, and everyone uses them, and it's practically unthinkable to want a mouse without one. I feel the same about my mapped paging buttons. I use the wheel for short-distance scrolling, but I use the paging buttons for wide-area jumping around in a scrollable UI element (e.g. web page). This particular mouse also lets me hold down the mouse button to get repeated "keystrokes" just like a keyboard.
I was curious what things other Tilders do with their extra mouse buttons. Maybe we'd pick up useful ideas from one another.