Great sound and great power, especially for something so small. Rivals my standalone desktop headphone amp. Due to sufficient power, the bass really slams when hooked up to my HD 58X's and the soundstage expands, both signs of good amp and good power. I also got it with the ear buds they were upselling it with. Not overly impressed with the ear buds, but the dac/amp itself is excellent.
UPDATE: Now that I've had this a little longer, just wanted to update my review. This thing is still freakin awesome. Sound is incredible, and a bit more "musical" than my desktop setup (laptop, SMSL DAC, Drop THX Amp) which also sounds incredible but has a bit more of an "analytical" sound. When I hook the ES100 directly to my phone (as opposed to LDAC Bluetooth), and use IEMs, it sounds just as good, if not somewhat better (due to more "musical" vs more "analytical") as my many times more expensive desktop setup. Just make sure you have the right music player software that outputs in true Hi-Res (I use the Sony Music Center app, but there are others). I know, technically, I'm not supposed to hear the difference between LDAC Bluetooth and direct connection, but I swear I do, although could just be in my head. But sounds great either way. Obviosly, it's not going to drive some hard to drive planars (I have my desktop setup for that), but it drives pretty much everything else I've thrown at it with ease. Highly recommend.