I have a 5-key mouse, and it's set up like this:
LMB - Fire, RMB - AltFire, MMB - Translocator/Camera, Right Wheel - Close Range Weapon (Flak/Bio/Link), Left Wheel - Long Range Weapon (Shock Rifle, Lightning Gun). I'm playing with the wheel a bit, earlier I had the default on, as up for the next weapon, down for the previous, but now I've decided to try out up for gravity-affected weapons (Flak, Bio, Assault Rifle), down for middle range weapons (Link, Minigun). Maybe that's not that good setup after all, because it limits me to 4 weapons + the number keys for selecting them, which is quite difficult... And gravity weapons pretty much equals close range ones...