For many years, the Kensington Iridio fit that bill perfectly, and Erica Sadun agreed. Yes, it was poorly-made, buggy, and many considered it really ugly, but I liked it and it fit my hand perfectly. It also allowed me to use the excellent Kensington MouseWorks, which is unfortunately no longer developed. That software kept me using Kensington mice. Once MouseWorks stopped being supported, I think with Mac OS X 10.5, pleas to Kensington got one incredibly stupid answer from them (paraphrased): Kensington mice work fine with the standard Mac OS X mouse driver, so MouseWorks is no longer needed. WHAT?! I could think of thousands of similes here; insert your own. In short, goodbye, Kensington.
So this freed me to find a replacement for my beloved, yet issue-ridden Iridios. I tend to like smallish mice, but no as small as the tiny mobile/laptop mice. I guess a medium-sized is how it’s described. Now the perfect mouse for me is the Microsoft Mobile Mouse 4000. It totally feels like an extension of my hand, and fills a more recent requirement of mine: side-scrolling wheel. That’s handy in so many situations that I find it annoying to use a mouse without. And the button clicks have nice muted sound, much nicer than the Logitechs I’ve tried and used. I do feel a little weird seeing the Microsoft logo here, but like stereo equipment, you go with the best for each component. If Apple made a mouse that were actually comfortable to use, I’d use it.
And the icing on the cake: software. Sure, pretty much any mouse will work out of the box under OS X, but I like to program my buttons and more detailed cursor behavior. I like SteerMouse for that. I can make my scroll wheel button paste, just like I got used to with Sun workstations, and I make that silly oddly-placed thumb button execute Enter, for once I’ve pasted something like a URL into a browser. And the developer is very responsive. I miss MouseWorks, but SteerMouse is a fine replacement for my needs.
This is such a nice setup that I might buy a bunch of the 4000s in case they discontinue them. In retrospect, I should have done that with the Iridios, but the hardware was so inconsistent that no two worked alike, so I’m best off not having done that. The 4000, in contrast, is near perfect.