Following up on this darkfield mouse (Logitech Anywhere MX):

It's been great for a long time, and I used it all day every day at work, both at my desk and in meetings, and its first set of batteries were still going strong as of a week ago. So, battery life is great on these things.

Gotten so used to the "Freewheel" scroll wheel that I hate to use anything else.

But its wireless dongle just crapped out on me. I was getting intermittent "USB Device Not Recognized" errors for a while, and they got more and more frequent until it just stopped working altogether. (On the internet, it says that powering off the system and removing the battery will clear those errors, but that's not the case here. This is a genuine hardware failure of the dongle, not a failure of the system's USB chipset. Confirmed this by trying the dongle on several other systems.)

On the good side, Logitech is being very nice about the return process so far. My first email to them received an immediate response, and the tech who answered the email, instead of just giving a boilerplate "reinstall the drivers" response, instead said, "yup, it could be defective. Please fill out the warranty information below..."