I dont think the logitech software can utilise all the capabilities of the mouse under PS/2, for example, the technology in that mouse is nowhere near what PS/2 was when it was first released as a connection medium with crappy basic keyboards and 2 button mice. Tracking isnt as smooth either under PS/2 with this mouse, even with PS/2 sample rate upto 200, the default 120 sample rate of the USB port still beats it.
Personaly I only the mouse for its accuracy, most of my shortcuts or key game binds are assigned to keys near to the wasd keys, the more buttons to press on the mouse in a game other than fire and duck, with mouse look just makes things messy. Just to illustrate the accuracy of this mouse, moving from a logitech mouseman dual optical corded mouse to the logitech has allowed me to move my Q2, Q3 & Q4 sensitivity from 8 to 2 and acheive the same movepseed and mouselook results. Its tracking has improved my rail ratio from around 60% to well over 80%.
However, with some games it has been difficult to balance the sensitivity, as some games are to low with say a sensitvity of 3, and too high with 4.
Overall the best mouse ive ever used. Period!!