I like it so far. As for improvements over the original Dragonfly (which I also have), jury is still out. So far I only listened to it in the combination pictured above (and this is brand new Spark, not my usual well broken-in piece). My first impression was that it made compressed and poorly recorded music sound better.
Over the weekend, I'll move it around to different configurations, where I can compare it with the original unit. And spend more time with headphones, of course. Dragonfly seems to work well with Grado (not surprisingly, it beats iPhone's headphone output by a wide margin) and I am curious to hear how will it work with Sennheiser HD650.