Everything had been working properly using v3.9.3 on El Capitan. No problem. Now, since the 10.11.2 update has come out, my mouse will often lose the ability to track, stopping the cursor dead in its tracks. Oddly enough, all the buttons still work though. Logitech's solution, as per their forums, is to unplug the receiver and plug it back in again. Oh, if that were a solution. It works again, for an indeterminate amount of time, and will often fail again, especially if my Mac has just been started up. Doing this also causes my button configuration to be screwed up; I have the thumb button on my Performance Mouse MX set up for Mission Control, and it stops working entirely after unplugging and replugging the Unifying Receiver. And occasionally, randomly, the modifier keys on my (otherwise unsupported by a driver) Microsoft keyboard swap their functions (I have them set to the proper Mac keyboard layout of Command on the right, Option on the left, using the OS X Keyboard prefpane), a problem which is often remedied by unplugging not the Microsoft receiver but the Logitech receiver. Huh?
This is an expensive piece of hardware (two, if you count my Microsoft keyboard) rendered nigh-on useless by malfunctioning software. Fix it!