I like my G5, it's pretty awesome and you said the price? It's like 10 bucks more than the 518 and has a nicer finish (satin vs gloss). I like how the middle button is raised more than my old MX500 plus I added some weight to it. A mouse that is only an epic failure? The Razer diamondback or whatever it is. I've gone through 3 of them, all of them died, biggest pieces of crap ever. The original one was awesome too, I got tired of paying for the shipping for replacements. I'd rather get the new G5 with 2 thumb buttons than waste more on any Razer product.
The other mouse I use is a Sun Crossbow (type 6) mouse, it's got a pretty high sensitivity, on par with my G5 at the half-way sensitivity mark.
I'm one of those binding whores, I have a binding for practically everything, raise/lower windows, minimize windows, popup menus, shade windows, etc.
People look at me strangely when I say I hate how apple laptops only have 1 mouse button, they reply "use the two finger click" ... doesn't help much when you need the middle mouse button.