I bought mine because I replaced my 2.0 speaker setup with a 2.1 with the volume knob on the sub, so for me it acts as a simple volume knob, but I've discovered other random uses throughout the OS.
It'll scroll through folders in finder, when in iTunes it adjusts iTunes' volume, in any other app it adjusts system volume, but one can easily configure it using the included software to have other scrolling or selection options in other apps.
Aside from the scrolling, the knob also 'clicks' so that can be assigned a function, like muting for the system, or something app specific.
Its sensitivity is great for my purposes, but using it in any other app, its just useless. I was thinking I might be able to use it to fly through long pages/docs, but the ratio between how much actual scroll you get between how much scrolling you're doing is laughable, even on the highest sensitivity settings. So you're pretty much boned for iMovie.
In addition, it has a blue LED on the bottom, which can't be shut off. When I was using it in Windows, the brightness of the LED increased or decreased depending on what the system volume was at, it doesn't seem to do it in OSX, despite how many times I change it in the software.
...any other questions?
