It's slightly thinner than my g500, but over all feels really good. The mouse 1 and 2 buttons are concave not convex, and give the impression they will work well in palm or claw mode (I felt in claw/fingertip mode the g500 wasn't quite right for mouse one).
The G light on it is annoying me whenever I'm typing, so that goes.
I'm not using the weights (didn't on my g5s or 500s either).
I think this will be easy to get used to from my g500 it's a similar shape. Not like the Razer Lachesis Banshee I had which was the worst gaming mouse I've ever used (personal opinion but whenever I pressed mouse one, the rear of the mouse would lift and the mouse would lose tracking!).
I also had a razer tarantula keyboard which was IMO rubbish before I got my CM quickfire storm rapid (cherry black) so I'm clearly not a razer fan.
The mouse isn't going to make me a gaming god, only a time machine and genetic resequence will do there, but first impressions are good.
Edit: The mouse wheel is so much better than the G5/500's ones.