This is an astonishingly useful tool -- I've been using it for years, and I'm always impressed that it's well maintained and getting new features.

From the way my music player (Clementine on KDE) pauses for a call, incoming or outgoing - the media controller from the phone, sms pop-ups on the computer, the file browser and two-way sharing, clipboard mirroring, and a nifty 'find my phone' feature (your computer triggers a phone ring).

There's a few features I've not used but are pretty cool to show off to 'other OS users' -- use the phone as a trackpad, triggering 'run commands', etc.