The special G keys on the G-15 are completely useless for WoW. There's only 6 of them. That means most of your keybindings will be elsewhere, and reaching over to them will take too long.

I use the old G11, with it's 18 G keys, and I use them for everything. Fingers on G7-G9, thumb resting on control. Easy access to 12 abilities using G7-G12 and CTRL+G7-CTRL+G12, and stuff I use less often on the upper set (G1-G6) and lower set (G13-G18).

I just bound the 18 G keys to number keys 1 through =, and NumPad 1-6, then used Dominos Key Binding to bind the G keys to action bars.

P.S. - That's exactly the point of this keyboard. The keys under the spacebar are modifier keys (SHIFT, CTRL, ALT, CTRL+ALT, CTRL+SHIFT, ALT+SHIFT, and CTRL+ALT+SHIFT). Since your thumb rests down there anyway, you would learn to use your thumb to press those modifier keys, while your fingers are free to press the WASD and number keys, using the new modifier keys to transform 1-5 and WASD into a massive number of different possible combinations. I wouldn't like the idea, since I'm a mouse-mover, but I imagine it would be pretty nice for some people.