Well, despite the fact I'm saving for an iMac, I went and coughed up the £32 i need to get this mouse off Amazon (that includes P&P).
It's a Razer Pro|Click Laser mouse, with a 1600dpi sensor, 7 programmable buttons, a gold-plated usb connector and the greatest ergonomic design i have ever used.
A quick photo:
There are two buttons on each of the sides, which accounts for the seven buttons (the mousewheel clicks down as well).
I have my buttons set as:
1st on left: Back
2nd on left: Reload
1st on right: Forward
2nd on right: not set.
I'm not using back and forward that much, actually, as mouse gestures in Opera are available through right click and a little movement.
The mouse is quite difficult to get used to at first, as it is so sensitive, although once you are used to it it's almost second nature. At full sensitivity, a mouse movement of an inch will take the cursor from the top left to the bottom right of a 1280x800 monitor, whereas my old mouse it was more like six inches.
Anyway, I highly recommend buying one, or, if you have no intention to buy a mac, get a Diamondback, as they're basically the same, except the Pro|Click works fully with OS X.