I have the original G15 and its definitely wide, just enough space on my pull-out kb shelf for a Ratzpad + G15. Also, the LCD isn't visible as its hidden under the desk if I use the wrist rest; the newer one should be better if you have tighter clearance with a pull-out drawer.
As for the G keys....I think the original 18 is probably too many but the 6 or whatever it has now is probably too few. Somewhere inbetween would've been better. On the original they're so far over that they're not really useful. Maybe in RTS or MMOs where keeping your fingers on the home keys isn't as important, but I found them largely useless in FPS games. Also, each cluster of 6 is identical so its hard to distinguish between them quickly.
The M1-M3 sets are also largely useless because they're so far from everything. There's really no way you'd want to quickly switch between them. They'd only be useful for maybe saving different profiles for different characters/builds for the same game.
What would be MUCH better was if there was a thumb D-pad or analog stick of some sort below the space bar that was mappable and allowed you to use that to shift between macro sets. Like up = shift, left = alt, right = ctrl, down = tab or something. So for instance, up+G1, left+G1 etc would all be switchable quickly and on-the-fly. I've been using this for quite a while with my various Logitech mice, but it requires programmable software and it doesn't always work well with every game. I'm really waiting and hoping someone puts this on a mouse below the forward and back keys. Logitech if you're reading this, you better give me credit if you use it!
In any case, $50 for either this new or old one is a great deal on a high-quality keyboard with a good feel and LCD illumination at the very least. If you can make use out of any of the G-functions and LCD that's a bonus, but I've found other (and often better) means to accomplish the same thing for the most part.