Okay, two weeks in, and I’m really liking G600. At the moment I have these commands set for G-keys :
G9 - invert selection
G10 - shift up an octave
G11 - shift down an octave
G12 - select all
G13 - select same pitch same octave
G14 - select same pitch all octaves
G15 - open/close MIDI editor
G16 - quantize
G17 - undo
G18 - alt key
G19 - duplicate
G20 - backspace
I have different quantize resolutions set for G-Shift keys. It’s amazing how much stuff I can do with just my right hand now. 
After two weeks of use, I’m very comfortable with having all these keys on the side of the mouse.
As for G510, a great thing about these programmable keys is that all the commands I program would be stored in the list in LGS on the left hand side. All these commands in the list can be assigned by drag & drop on to any G keys. If I don’t like one layout, I can just shift things around very easily. I’m soo happy with this keyboard.