I received my G13 few days ago, and I am VERY happy with it.
Beautiful piece of hardware, it looks very well built and feels solid. It is quite heavy and does not move on the desk.
It is very comfortable. Keys action is good, the two thumb buttons next to the stick click nicely. Clicking on the stick itself is a bit too tough, it is hard to do it without actually moving the stick. I was afraid the stick would be less convenient/ergonomic than the D-pad on the N52, but that's not the case. It took me only a couple of minutes to get perfectly used to it, and it is in the end less straining. It could be difficult if not impossible for small hands to reach all the keys without moving at all, but I guess that's the price to pay for more available keys.
I use mainly one of the three profiles, with shift and control on the mouse side buttons to switch action bars in the WoW interface, just like I did with the N52. An other profile is simply used for actions that are not performed very often (keying in password, PiP switching, group forming, group disbanding, porting back to the inn, etc ...).
To me, the screen is fairly useless. I turned every program off but the one showing CPU and RAM occupation.
The extra keys really make a big difference compared to the N52. No more action bar switching during a fight.
At 89 euros, it was certainly not cheap, but I think it was well worth it.