I bought it brand new, but there are a lot of things that I had to adjust to. The g500 has an oddly placed sensor (it's not centered) compared to the g400s's sensor. The g500 has built in accel that, sadly, after two years of use, is pretty much engraved on my muscle memory. I was going to install the QL accel driver onto it, but I'd have to change off my weird-ass DPI to a native one. I'm kind of a wreck when it comes to mice.
Anyone else shouldn't have any problems with it besides the natural adjustment to a new mouse.