Ok, have had Logitech mice for a fair while as a gamer, last mouse before this was the predecessor, the Logitech G5 gaming mouse. Was a good mouse, pretty good response and was basically a big comfortable mouse.
The G9 has some pretty good abilities under the hood, the 3,200 dpi is pretty sensitive and may take some getting used to for those fast reaction FPS games but it also has the same variable resolution buttons on the as the G5 (very handy).
My only concern is the shape of the G9. Most mice are based on the "coffee bean" shape, named smooth and curved. The first thing you notice when holding the G9 (especially with the larger interchangeable mould) is that the back is almost square cut.
When you get used to it, it does make for a tighter fit into the hand, but even after a few days use, it does still does not "feel" right in your palm.
Conclusion, is a great performance mouse, a good evolution of the G5 but the shape may make this a "marmite" mouse (e.g. you will love it or hate it).