My HP50's arrived today! Here are my quick early first impressions. I'll update if my impressions change.
Soundwise, I really think Paul Barton really nailed it with these headphones, and I'm a picky bastard. They have a fun and aggressive sound without relying on the standard v-shape crutch that a lot of other "fun" headphones have. Instead, these headphones have low distortion, very little reverberation, and a very capable driver with great transient response. This leads to an aggressive and punchy sound that also sounds balanced and doesn't have any noticeable dips or inconsistencies in the frequency response. A lot of people seem to treat balanced and fun as opposites, but the NAD (among some other headphones) show how fun balanced can be. These headphones respond very well to EQ, because of their low distortion numbers you can EQ these guys a decent amount without having to worry about things getting out of hand. Personally, I've been using the "slight" bass boost setting in Cyanogenmod 11's DSP Manager (which is surprisingly awesome). Normally I'm not fond of bass boosts, but the combo of the good quality boost found in the DSP Manager and the low bass distortion of the HP50's leads to a very nice result. I also tried out the "virtual room" in the DSP Manager and it also worked great, the headphones sound was altered, but there wasn't a noticeable quality loss. The virtual room increased the soundstage a bit, and smoothed out a bit of the aggressiveness. The soundstage itself isn't as wide as I expected after looking at the measurements, but it is pretty decent for a closed headphone.
If I had to make a complaint about the sound, I would say two things. It does have a synthetic quality to the sound, but every closed headphone in this price category that I've heard has that sound, and the NAD has it the least. I would also say that the sound is pretty aggressive at times. I'm not sure how that aggressiveness will stand up to 10+ hour marathon headphone sessions. I'll try to fit one of those in within the next week or so to see.
Build quality is decent, not up there with some others like B&W, but these sound better, so I don't care much. I have the white headphones which look great and don't show fingerprints, but does seem to attract small pieces of lint. Also, the cup swivel mechanism on the left earcup creaks.
Comfort is more of a mixed bag. The earpads are about as small as possible while still being successfully over-ear. They're very plush and comfortable, but my ear touches just about the entire inner part of the pad, including parts of the back/baffle. So far it hasn't been an issue. I'll update if it is. They're also very hot because of the pleather and the very good seal. I'm not sure how comfortable these will be in the summer hear. The headband is really not great. It has very very little contact with your head because of the square shape and the fact that it is shaped like a bent pipe. There's a fair amount of padding, but it's all focused on one very small part at the top of my head. To top it off, the headband rests exactly at the peak of my head, focusing the pressure on an even smaller part of my head. I can move it forward to a more comfortable spot, but then this adds pressure on parts of my ears. I'm not sure which I find less comfortable yet.
TL;DR - Super awesome sound in a somewhat uncomfortable and "eh" package.
Well that ended up being longer than I expected!