This mouse is extremely sensitive to any type of texture in the surface it is used on - wood grain, fabric mesh etc. The problem appears as follows:
As I move the mouse right-to-left the cursor moves down as well as left. The ratio depends on the angle of the grain in the surface you're using it on with respect to the horizontal movement of the mouse. On my desk the very light wood grain is at about 10 degrees to the mouse horizontal, so the mouse drifts vertically by about 1-in-10 as you move horizontally.
It's an insidious problem: far from obvious. It took me about a month to become conscious of the mouse zig-zagging as I dragged horizontally: it was me compensating for the vertical drift by pushing up or down briefly, then repeating as I moved further horizontally.
As a result targeting with the mouse takes longer and takes more effort. Now that I'm aware of it, it's bloody annoying.
Logitech themselves suggest using the mouse on a white surface, and a test on plain paper seems to rectify the problem ...more or less. But it's hardly what you'd expect of a multi surface optical mouse. I need to pick up a plain white mouse mat from somewhere, but overall I definitely prefer my older, wired Dell optical that this was supposed to replace.
In all other respects this mouse is perfectly fine.