I've been using the Logitech MX 518 for about a month now. It's been a huge upgrade over the ordinary Logitech USB mouse I used before. It's very responsive and has very good tracking.
I recently bought the MX518. I cant compare it to the G500 because i have never used that mouse. But this MX518 is very nice indeed. So much better then my old mouse.
I have the G5 (predecessor to the G500 I believe) and the MX518 here. The G5/G500 is worth the few extra bucks if you have it. The shell has somewhat of a matte grainy texture that feels like it's of a better quality than the MX518, which is kind of glossy/plasticky. The G5/G500 is also easier to grip in my opinion.
Extra buttons aside, the G5/G500 ships with a customizable weight cartridge, and putting a little weight in the device adds to the robust feel. Performance-wise, I'd say they're pretty much identical to me, it's just the surface features that change the experience. Both are awesome, and I can comfortable recommend either.
Bit off the subject but I'm looking to buy a new mouse but want one I can lock the Axis on. In otherwords the mouse has the facility to travel horizintally or if you prefer vertically without going off a staright line.
As a native hardware function? Not that I'm aware of. SetPoint (Logitech's driver/software) might offer something like that, but I haven't checked it out in many years so I can't say offhand.