hmph! I bought a G5 because I thought it was one of the best out there for the money. No i think I'm regretting it (only in comparison to the other mice anyhow).
I've had it for a little more than a month, and have never experienced a problem with it, I like the weights and the adjustable DPI is a feature which I really do use. The mouse's feel is nice once you get used to how to hold it, but other than that, it's a downright great mouse. Now I think I could get a better mouse for less money, but I don't reall figure like sending it back to newegg, because it does work perfectly well.
The MX518 is a very nice mouse, I have used them, and never experienced a problem with it either (it was at a LAN party, so I was using it for a long time straight). The G5 moved prematurely ONCE, but only a little to the left, and ended up aiming at an enemy player in AA.... so it wasn't much of a loss lol.
If you don't want to waste money on the G5, which is very reccomended now that I see the comparisons, I think I would go for the MX518, because the MX500 looks a bit odd, and the 518 is an ergonomic mouse, with no tracked malfunction point (> 4.50m/s).
Anyhow, all in all, I reccomend the MX518, because I love the feel, it never malfunctions and has quite a few buttons. Plus it looks hawt.