As a long time g500 user who recently switched to the g400, please allow me to give some input.
I bought my g500 a year and a half ago. The mouse I had before it was a deathadder. My reason to leaving the deathadder was simply that it was too small for my hands.
I loved my g500 for the longest time. But over time, I grew tired of the weight of the mouse (heavy, even without any of the weights), and the positioning of the sensor as mentioned before.
About the placement of the sensor and its alleged cause of acceleration.
The g500 indeed has really weird placement of the sensor. If you tend to use your wrist for a lot of mouse movement, your left and right movements are greatly exaggerated compared to mice which have the sensor placed in the center of the mouse.
However, this does NOT induce positive acceleration. The flaw of acceleration with the g500 is indeed present, but this is an inherent flaw in the actual sensor itself. Mouse acceleration is a result of electronic/software design, it is not a physical entity that can be blamed by where the sensor is placed on the mouse.
Like I said, the sensor location on the g500 is definitely strange and causes certain mouse movements to be exaggerated, but it is not the direct cause of accel, it is the sensor itself that does this.
All that said, I'm now a g400s user. I've had it for 3 days now, and I'm already certain, without a doubt, that I made the right choice to switch to the g400s.
A little secret. The only honest reason I waited so long to ditch the g500, is because of the amazing extra thumb button. The g500 has a button that sits between mouse4 and mouse5. The position of this button, the force required to press, and the contrast of its surface compared to the mouse4/5 buttons, is amazing, and I mean perfect, 100% the best button for your push-to-talk needs. It really is that amazing.
https://dl.dropboxusercontent.com/u/43946806/Mouse-2.png
On another note, about logitechs customer support.
You may hear people talk of the build quality of the g400/g500. While I can't honestly say they are built to the greatest standard, I can say that logitechs support is incredible.
My first g500 eventually had cord problems. Logitech sent me a replacement for free. I didn't have to send them my broken mouse, I didn't have to provide proof of purchase, I didn't have to send them pictures. I just gave them info, and they sent me a new mouse.
My second g500, the one that replaced the first one, also had eventual problems with the cord. Logitech sent me a third one, same thing, no hassle, they just gave me a new one.
My third g500 just started doing the same thing. I told logitech I'm tired of the g500 and want a g400s. They sent me a g400s.
Just saying, you can't beat that level of customer support.