Clean, smart and cross-platform. Very impressive.
Easy to use, REALLY easy to install, and feature-rich. Very quick responses from the creator (who is alive and not missing-in-action). I've tried a bunch of APRS clients and this is the one that has made me the happiest.
I've been using this for some time. I've tried just about every APRS client for linux that I can find and I end up coming back to this one. It has all the features I need and is easy to navigate and setup.