Yeah, I'd get the g400 if I were you, I've had an mx518 for years and currently have a G700 as well. I like the 518 more but the 500Hz hack doesn't seem to work for me.
The G400 is the same shape exactly as the mx 518 and presumably the mx700, the differences are it goes to 3600dpi over the 518's 1800 (care), it does 1000Hz out of the box and it looks a little different.
The G500 feels slightly different but if you can get over the acceleration it's a very nice mouse as well, the G700 is a different shape but again feels nice enough.
The thing that gets me is the acceleration on the G500 and G700 is very noticeable, it doesn't affect my scores in games but it gets on my nerves when I'm gaming.
Hope this helps, it's all subjective though so you should try out the mice before you buy if possible.
Out of the three I have I'd take the 518 any day if I could get above 125Hz to actually work with my current setup and I reckon I'll get the G400 when it comes out if it's not overly expensive.
The G500 and the G700 are pretty much the same mouse, just the 700 is wireless and the 500 can only store 1 profile, correct?
Yeah prettymuch, but they're different shapes and the 700 has a few more buttons. The G500 is closer to the shape of the MX518/MX700 than the G700 is though.Edited by Mike-IRL - 6/20/11 at 8:33am