I still use my Kensington ADB trackball on my G4 (picked up a USB/ADB converter). Nothing finer than the older ones - the newer ones just don't seem to have the roll/mass.
I like the older (ADB) better than the 'middle' ones (USB w/6 function buttons), but the one pictured is sweet. I have it on my computer at work. It's an optical trackball (no rollers to clog) with a scroll wheel that works swimmingly.
The feel is very close to what I remember from my old ADB Turbo Mice.