Cons: Sensor, Build Quality
The sensor is junk, it stutters around as if moving on a pixel grid no matter the DPI used and strangely it fixes if you unplug and replug it in. I wont get in to the other well documented issues the crap sensor has.
But my main issues with it arn't really the sensor, but the way it's built. On the side there are the browser forward/back buttons and if you take it apart (I did) you see the it is a flimsy piece of crap. What ends up happening if you actually use these regularly is that it will break, since the switch is not directly under where you put pressure on the button.
On top of that the crappy soft-touch coating, while nice initially, is very "fragile". If you have finger nails of any length or you use a claw grip you will probably end up scraping the coating off.
Then you have the thumb wheel, which you will never use. If your hands sweat it will get corrosion, which is gross.
I mean it is a fairly solid mouse otherwise and I get things wear out, but at ~$100 it should last more than a couple of years.