Mine came in a few days ago...and I'm returning it. It just wasn't as good as my mx500 for me. There's no mouse orientation option in the new SetPoint software, and for me, that won't do. My wrist goes down to the left and up to the right when I use the mouse, so with no orientation, it made it very difficult to use. Also, the left side buttons weren't as easy to click in my opinion; they were all usable, but not as easy as the 500/510 series, and since I use them an awful lot, it wasn't favorable.
The other major issue I had was again with the SetPoint software; it had less features than the previous mouseware software...two of the notable ones were minimize all and maximize all, which I use hundreds of times daily.
The feel of the mouse was better though, and the weight wasn't at all a problem. It charged fast and was pretty nice save for the few annoyances I mentioned. The horizontal scroll is a very nice feature, but it wasn't a worthwhile switch for me.