My DT250 sounds fine through it - no obvious flaws, very clean sound, but it was only middling with my old K701, and pretty crappy when I tried an HD650 through it. Definitely not good with high impedance cans. Re IEMs, there is definitely an impedance mismatch with my TF10s, but my iM616's sound really good through the ULN2 - despite slight sibilance. It seems good with lowish to mid impedance cans. My old MS-Pros sounded pretty darn good through the ULN2, as did AD1000.
I've never compared it to a desktop amp but hope to do so soon. But with the right cans, I can already tell it has a nice sound.
Of course, the high quality mic-pres and DA/AD make it excellent value for Mac users who are looking for those features also.