Well, not sure about your rebate Cheapherk, but the one for this G9 is supposed to be mailed to Logitech it looks like... so hopefully I have more luck than you seem to be having.
Anyway, just got done playing a bit of Portal, Crysis, and TF2 with this mouse, and loving every second of it. The "on the fly" DPI switching is great, and they really let you fine-tune your mouse through the setpoint system. Sure, my MX518 had buttons that allowed me to change the sensitivity in-game, but it never seemed to work that well for me. Maybe I just never set it up correctly, who knows.
Also, I love the way this mouse feels over the MX518 as well, like I've mentioned before, it's a lot flatter, which is what I wanted. I've only tried the default, rubbery shell, but it feels fine. I haven't messed around with the weights yet either, seems plenty heavy enough to me with nothing in the mouse. I'll have to read up on what adding weight to your mouse can do for you.
My only complaint about the mouse would be that the wheel click feels a bit too "heavy", I really have to press down on the wheel to make it register. I'll eventually get used to it I'm sure, but coming from a MX518 it needs a lot more pressure to make it click. I do love the "Hyper-fast" mode of the wheel, which removes the click-click-click rotation of other mice. The wheel is weighted, so when you give it a flick it spins and keeps on spinning, making the page scroll by as fast as you want it to. I can get to the top of this page (takes me 6 page-up's to get to the top) in about 1 second if I give the wheel a flick. It's easy enough to switch between the two modes and have your wheel actually click when you rotate it.
If you're looking for a good gaming mouse, and would prefer a slightly lower profile mouse (flatter), then I would highly suggest you get your hands on one of these and test it out. Awesome mouse.