I've got a G5 at the moment and I'm considering getting the G9 which has 3200dpi vs the 2000dpi of the G5.
The other thing is that the G9 has got the extra button on the left side. The new G5 has it as well but my G5 is the older model which didn't have it. That can make a big difference depending on the games you play.
Another nice thing is that the G9 saves your game settings on onboard memory (up to five game profiles).
In the end I'm waiting till I can hold a G9 in my hand, then decide whether I want to get one or not. The interchangeble grips are interesting but I think it's a bit of a gimmick. Altho, I thought the ability to change the weight of the mouse was a gimmick as well, but that actually wasn't a bad idea.
Have a look at the differences between the G5 and the G9 on Logitech's site.
I've done my research... definitely going for the G5. Looks better and according to other forums has a better feel to it. The G9 has more like a Razor feel.. I had a Razor Diomandback, didn't like the grip at all. Anyway no one I know uses more than 2000 dpi so anything above is overkill. Thanx!