Thought I would add a few more comments as I just received my G9 yesterday and spent about 3 hours playing with it last night. I will touch on a few aspects of the mouse that surprised me. Take in to consideration that I'm a competetive CS:S Junky. CAL, CEVO, ESEA.
Weight System - I've always liked a heavier mouse---one of the top reasons I ordered this particular mouse. I loaded it down with four of the 7g weights, threw the cover on it and went to town. The thing is HEAVY or it can be very light...or anywhere in the middle. Please, in your consideration for purchasing, understand that you can tailor this thing to the exact weight you desire. I doubt there will be a person that says, "It's not light enough" or "I can't get it heavy enough"
Wide Design - If you're a player that uses low sensitivity and picks up your mouse alot, you may have trouble adjusting. The wide base offers a good feel, but rather chunky and uncomfortable to pick up and scoot. Also, if you are a "scooter" that normally just picks up one edge and moves the mouse, dragging the other side, to relocate without registering any movement on screen, you have to relearn your habbits. The wide grip makes it difficult to "tip" the mouse up to drag it. Same concept as a wider based vehicle being hard to flip over.
Scroll Wheel - I've seen free spinning mouse wheels, but never like this. When you disengage the click mechanism the mouse really does spin freely and has a great momentum. If you give it a good spin it will remain in quick motion for aproximately 10-15 seconds. If you bind your mousewheel to make something easy for you (like bunny hopping in CS)...this thing is amazing. I can bunny hop like a maniac with it.
Precision - That's all I'm going to say. I noticed an immediate difference between this mouse and my MX518.
Overall, I enjoy what I see so far. It will take some adjusting to find the sweet spot for this mouse with such a great increase in precision, but remains a good move if you're a gamer. Buy from an online store to save about $20.00.