I own a G500 and it's the best thing I've seen since sliced bread. Not so much for gaming though.
The features are what do it for me: the unlockable scroll and the three side buttons are fantastic in day-to-day computing, browsing the web and whatnot, it speeds up everything if you configure it to your liking, but in games, it's all about the precision and the feel of the ting. I'm used to it and it's very precise, very dependable, so it's good, but games are DESIGNED with the barest of bare mouses in mind, so you won't get any tangible advantage out of a top of the line mouse, unless it's something like that wierd ass 16 button mouse I keep seeing around, and even then, probably only on MMOs or something like that.
I owned a Microsoft Intelipoint 1.1A for ages, and it was very reliable, very ergonomic and a peach for gaming. When my second one died, last year, I bought a 15€ Microsoft mouse, and it had a slight delay on the middle click. Which means it's useless. And since I kept reading Microsoft mice are crap nowadays, I went with Logitech.
They are good.