Just downloaded and install PS1, not the easiest of things to get working. Had a runtime error so had to put planetside.exe into xp compatibility mode, and even once I get ingame the mouse sensitivity is unusable. In menus the mouse is super fast, and when not in menus it doesn't pick up fine/slow movements of my mouse, and if I speed up my mouse just a little it 'jumps'. at high mouse movements it doesn't seem so bad, but for precision stuff it's unplayable until I can figure out what's wrong.
I found the mouse to be dodgy as hell as well, played a bit with setting and got it OK now, game runs OK on my system now.