I have only used this for a few hours so far so it very much a case of 'first-impressions'. First off, I was really disappointed to discover that this mouse never switches its red LED off, even if the computer itself is switched off, only at best decreasing LED-activity, - so that little on/off switch on the bottom really is essential! My disappointment stems from the fact that I've been using a much cheaper Tevion mouse for about 6 years that would switch off after a pre-determined period of inactivity, switching back on when one of its buttons were pushed - a much more sensible & eco-friendly solution, especially given that Logitech use the power-saving angle in all of their advertising. In fact it makes no sense doing it this way - the only reason I can see for this is it's a 'Micro$oft-like' ploy, hold-off including even basic-features (like this!), but including them in more expensive follow-on products! But I guess the mechanical switch should have been a give-away...
The other reason I'm only giving this 3-stars has to do with the size - it's just too small & light, at least compared to a full-sized mouse, which it most certainly isn't. And although my hands are my no means huge, after playing just a couple of games of Majong, the muscles in my hand start to ache from 'cupping' this little mouse. But like I said, maybe this will pass after using it for a bit.
On the plus-side and definitely better than the Tevion mouse, it is extremely responsive - maybe a bit too much! - so its resolution is much higher than what I'm used to. In fact with 'Acceleration' etc. set to minimum on my setup, it is still a bit too responsive for my liking.