Hi Martin,
I use Macs for my Pixinsight processing, and I've had good luck with a Logitech M555b bluetooth mouse. This has been a really useful mouse for me.
The zoom wheel can be `click-depressed' in order to toggle between a free-spinning, flywheel-like scroll mode (very handy for quickly scrolling through big webpages or PDFs), and a detent-by-detent scroll mode. This latter scroll mode allows me to go from one PI zoom level to another very precisely. I tend to use the free-spinning mode when I'm in programs other than PI, and the detent-by-detent mode in PI. I'm glad it's easy to switch between modes, too.
I use Expose and Spaces a lot (on my home machine, running Snow Leopard), and a simulation thereof called TotalSpaces (on my work machine, running Lion). The tilt-left and tilt-right functions on the scroll wheel are very useful to me when using Expose and Spaces.
The fact that it's bluetooth means I don't have to tie up a USB port with a mouse-signal receiver.
- Marek