I've been using the G604 as my secondary mouse mainly for playing LoL and other clicky games. Great mouse overall and no noticeable wireless lag.
The only thing I don't like is the mouse wheel. It is one of those very heavy metal wheels, so poor grip and hard to spin. It does have a free spin mode however I don't see the point of it as it spins too fast to be of any use for me, I have read that other people find it useful though. It can do side to side clicks which I like for going Back and Forward.
Other than that, good shape, side buttons are perfect, standard mouse clicks are nice, not too heavy, runs on a single disposable AA battery which lasts months (which i prefer over built in Lithium Ion as these degrade noticeably over a few months/years and end up needing to be plugged in all the time, i.e. pointless as a wireless mouse).
I would rate this 7.5/10.
My primary mouse is the Logitech G305 which is a lighter mouse and position of the sensor means its better for me for playing FPS. I would rate this 9.5/10.