Ardour if fantastic. Its mixer is better and more usable than pretty much any other DAW I've tried. They are competing with software that costs hundreds of dollars and winning in many aspects. They should be an example.
Another thing I like is the routing grid. So flexible. Along with Jack, this does add some complexity and sometimes extra manual configuration, but the expanded possibilities are worth it for me. I have yet to encounter a patching scenario I can't handle with Ardour and Jack. Including things like playing an Ardour session over Skype to a music teacher while doing real time effects processing on my guitar all while recording the audio of the Skype session. Ardour + Jack is a powerful combo.
Absolutely. Another added bonus: it is really easy to use Ardour as a mixer and effects plugin host for experimental synthesis software like Supercollider or PureData