They compare excellently with mostly anything. Don't let the price detract you from thinking they're any less capable. Ok so the power output isn't as high as some competing models, but they're pretty much capable enough to power basically anything at safe listening levels. I use them to power my T1's, HD800's and Z1R's, and there's more than enough juice for me.
Ultimately though, the noise floor is zilch, and the audio quality is superb. If you asked me which was better out of my £380 HP-A4BL, my £690 Violectric V200 and my £230 O2+ODAC, I wouldn't say either. They're equally as capable as each other, but have different strengths and qualities. Audio quality wise the V200 is a touch coloured which makes it fun to use, the Fostex and O2 are essentially dead neutral (at least when the HP-A4BP is connected via USB). The V200 is the most powerful. The O2 the cheapest but least convenient and worst looking, and the HP-A4BL isn't nearly as powerful as the V200, but is still powerful enough for me, whilst also being small, nice looking, relatively affordable and feature rich, thanks to both optical and balanced options (I'm predominantly using balanced now, with both my Z1R's and HD800's).
If anything, the most annoying thing about the HP-A4BL is that it doesn't have a power button lol. I dislike having to seek out the ideal volume level each time I use it, because the volume knob is the power on. I wish I could just leave the volume knob at a certain turn, and just have a switch to turn it on and off.