I paid $80 AUD for mine from the Logitech eBay store!
I love it! The only thing i don't like is the excess of "G Keys" i don't use them at all personally..
The screen is great for CPU/RAM usage, the Time and music! It'd be good for emails too, but personally i couldn't be bothered.
I like the RGB backlights too, along with the "Gaming mode" which disables the Windows keys, the modes are cool but kind of useless to me once again.
The play/pause/stop etc is nice, those are soft touch buttons, the volume control is nice and smooth and it feels like you have precise control over it. The backlight on/off is a nice feature too. To be honest the only thing that doesn't work all the time is the "headphone" jacks.. It works half the time untill i use the mute key, then it won't unmute at all... probably software conflicting though.
As for software, it uses the same software as my G400 which is a nice touch, not having to worry about different software. I guess I'm a logitech fan which works out well. (It matches my G400 perfectly)

Keeping in mind the G510 has a massive palmrest.. I took it off recently to clean it and it just looks so much more compact, I couldn't stand not having the palmrest on it!
Oh, and as for Mechanicals, I like them.. But i'd prefer something more silent myself, the soft mechanicals cost loads too. (In Aus anyway).
P.S people say the screen is a "waste" or "not useful" but you get used to it, I went to a friends house with a G110 and i was lost!

If you decide to get the keyboard it's a great deal! If not be sure to grab a good deal on a mechanical!
