My previous mouse, an MX500, worked great, and when it gave out after a few years, I replaced it with this one, bought from another retailer. It was a reasonably good replacement. After about two months, the left mouse button developed doubleclickitis, meaning that sometimes it would click once as intended, but mostly it would doubleclick. I had long tossed my box and receipt, so I couldn't return it.
It is a good mouse, although not as comfortable as the MX500, but you can get the occasional lemon.
Update: I found a video on Youtube that showed a very simple way of correcting the double-click problem: Simply slam it on your desk. I would not recommend this for a flawlessly working mouse, but if you're on the verge of replacing it, you have little to lose. Since I tried this remedy, my mouse has been working very satisfactorily.