IÂ bought this mouse to replace my Corsair M65 as IÂ found IÂ could do with a bit of added funcionality due to the large range of games IÂ play.
So far IÂ have found this mouse amazing to use, every button other than left click is programmable, meaning you can set it up to match exactly how you want to play. IÂ have found it is rather easy to hit the buttons on the outside of the mouse if you aren't careful, but a slight change of grip fixed this problem very easily.
I have read reports of tracking issues this mouse can have, so I recommend reading into these before dropping such a large amount of money on a mouse. Though I personally have experienced none.
As far as comfort goes, this is by far the most comfortable mouse I have ever laid hands on. While you are only able to adjust the length and the angle of the palm rest, this is by far enough. It is comfortable in both palm grip and claw grip, although depending on the person, you may find a part of the mouse sticking into your hand slightly (Can be a major drawback depending on how your grip.)
While this mouse does have drawbacks which are far better documented on review sites than I can put here, I, personally, give it 5 stars and would recommend it to everybody. This is the first Razer product I have ever owned and by the looks of things, it won't be my last!