I have one of these mice on each of my two machines. I really like mine, they look good, cordless, optical and the batteries running out is just not an issue. :)
As for the USB Vs PS/2 question - I'm not sure but doesn't it have to use the USB port in order to supply power to the charger/receiver station? =-s
I do use mine for gaming, but TBH it's not as good as my previous cordless Logitech mouse that had a ball rather than being optical. Personally I think the other benefits of the mouse are worth the trade-off and I suppose I could switch back to the old maouse for gaming if I really wanted to.
Hope this has helped. :)