I've had a few days with the Logitech G600. My thoughts are thus:
-"You don't need all those buttons!" Oh, but it does improve things. I never have to leave the movement keys. It also makes FKey usage so much nicer. I can pull off quicker shatter combos now that it's easier to transition from skill chain into a shatter.
-The buttons are pretty easy to tell apart. They are laid out in rows of three, with different inclines to their angles, making it easy to feel different rows from each other.
I would say the lower two rows are much easier to reach than the top row, but I normally play with a mostly palm grip. If I go halfway between palm and claw, I can access the top row fine.
I don't normally use the buttons towards the back of the mouse and just use a shift modifier and mouse's modifier key with the the first six keys to have like 18 different actions at my thumb's command.
-It's a real good size for medium to small hands. Might be too small for very large hands, but the G600 is bigger than the mouse I had been using.
-My criticism would be that it's really easy for me to accidentally hit right click when I mean to hit the modifier button. The modifier button is placed right where your ring finger naturally lays, but since I'm not used to using that finger often it can sometimes screw me up. The modifier button is also rather sensitive compared to the left and right click.
Overall, I really like it.