Within the last 30 days as I got into multiboxing and built a rig I used a G403 initially. I just got the G600 in the past two weeks.

As a nearly 50 year old, with large hands and mild arthritis, here is my feedback;
* The mouse is actually very comfortable, minimizing strain on my thumb (I am a "Claw" mouse user)
* I personally like the "3rd" button on the right top (ring finger)
* The mouse wheel in addition to a "push" in button, has a side left and side right action that is also very useful
* The mouse has more weight than the 403 (which I like)
* I far prefer Logitechs key map software to Razer

Only downside for me
* It is still difficult for me to distinguish between the thumb keys solely on tactile feedback. I find myself using the "corners" to avoid mis-clicking on the other nearby buttons

My two cents