The weight is nice when you are playing at 3000 dpi or better. It prevents inadvertent vibration from spoiling aim or movement. I can play my Rat 7 at a much higher dpi than any other mouse I've owned. Once you get used to the weight, a lighter mouse feels like a balloon, all shape and no mass.
The easy dpi shift allows me to sufficiently lower the sensitivity so that my wife can use my computer without having a fit. She never got used to my Logitech G9.
The main issue to me is that this is really the Ferrari of computer mice. A small change in configuration can cause a large change in sensitivity or handling. A N D . . . since the Rat7 is almost infinitely configurable, the urge to make a "small" change now and then is considerable. For FPS games, it is a real asset. The sniper button is a masterstroke, which I expect to see copied by other manufacturers. I usually play between 3 and 4000 dpi and the sniper button is set to lower this to just 10%, making scope shots almost unmissable. The first time I went online to one of my haunts, I was warned off by the server for using an aimbot. Had to explain my new toy to a mod.
It is truly said that once you get used to a gaming mouse, regular ones feel like a piece of inert whatever, and you will not want to go back to a regular mouse. I have agreed with this for years, though I usually stuck with Logitech products.
I gave more than a bit of thought to the purchase of my Rat7, but I realized I'd spent almost as much on my Cyborg Steel Series Merc Stealth hybrid gaming keyboard. I play enough online FPS games to make them worth my while. They don't really give me a large competitive advantage. Afterward though, I can still feel my fingers, wrist and move them freely.