I've been using the mx 510, mx518, g400, g400s, the form factor is just perfect. This was further confirmed when I was shopping for a new mouse recently. I bought the g502, it was just horrible, the form factor is the same, but the mouse is ruined by the new features. The grip on it creates sweat. The 1337 h4x0r sniper button is just where your thumb rests so you're always accidentally clicking it. The scroll wheel is slippery as hell. The buttons above and below the scroll wheel have been moved to a very awkward position (these buttons are great for home and end while scrolling GH and always going into posts you're watching and want to go straight to the end of the post to see the newest reply). The look has also changed to a obnoxious gamery look where it looks the mouse is stitched together with a lot of parts which look and feel they could all fall apart any second.
I also tried the Zowie EC-2A. It was OK. Resembles the ms 3.0 which was awesome in the old day for cs. But that's just it, the mouse is good for cs, because it's good for palm grip which is good for fps. But claw/fingertip grip, which i use for Starcraft II, is not good on it. The side buttons are also very bad. Maybe the ambidextrous offerings from zowie are better for claw grip but they will still feel cheaper than the classic logitechs. So I learned my lesson that the Logitech's mentioned above are classics. I still want to try Logitech g9x, they might be good, but discontinued.