- the changing backlight thing is a complete gimmick
I actually find this very useful, I switch between a few different colours regularly, some ear easier to see in different lights
- the keys feel a bit mushy
this is the biggest problem I have, they do feel a bit mushy, although I'm used to them now after 5 months
- i didnt use the macro keys whatsoever (what would one use them for in bf3?)
I find them more useful out of games, for instance using them on forums to input image tags etc, saves a lot of time
- the screen was useful for watching cpu and ram, but im indifferent about not having it
I find it great for the cpu/ram usage as well as core temps.. I could live without it but I find it very handy
- the volume scroll wheel is useful
agreed, it's nice to use 
- the audio port extensions are useful (bare in mind when you plug/unplug the audio the keyboard restarts and the computer sees it as a virtual sound card)
these are gread, and I swear they're amplified too, plugging my headphones in, I have to leave the volume on like 10% max!
I moved from the g510 to a filco majestouch with mx browns and i have not looked back ever since, the g510 felt like a bit of a toy in hindsight
imho: get a mechanical board