Looking on my desk right now, I've got (roughly arranged in order of preference)...
Kensington Mouse-in-a-Box Optical Pro
Apple Desktop Bus Mouse II
SGI three-button charcoal USB mouse
Apple Mighty Mouse (oddly enough, being used with a PC)
My favorite is probably the Kensington - I bought it years ago to use with my G3 iMac and use it on my G4 now, so it's the mouse I use most often. The ADB Mouse II is the most comfortable, but it's not as practical as having a few extra buttons. The rest are very good, although the Mighty Mouse has a few quirks (I only ended up with one because a friend gave it to me, but I do like it). There's something I like about the feel of a one-button mouse, so being able to have that and also right-click is useful to me. The shape of the SGI mouse fits the hand incredibly well, making up for it not having a scroll wheel.
What about this IBM mouse? It looks nice, and I was thinking about getting a "proper" PC mouse for my PC so I can free up the Mighty Mouse to carry around with my laptop. I'm a stickler for aesthetics, like IBM stuff, and am already using an IBM keyboard with the machine, so... anyone besides Scott have thoughts on it?