edit: This was originally about the Senn 350s, but it looks like the sale is broader than I'd realized. Read the thread for more info.
I went shopping over the weekend, thinking about replacing my $10 headset with a better unit, and spotted that Amazon has the Sennheiser PC 350 (retail $250) at $120 if you add it to your cart. So I picked up a pair.
Initial impressions: they come with just the headset. You don't get anything else. The PC-151s I bought a few years ago came with a USB headset adapter and a cable guide... in this case, you get the headset only.
They're closed headphones, but they're completely circumaural. I suspect you could wear them for reasonably long periods, although I've only been experimenting with these for about a half-hour. They clamp fairly tightly, so I don't think they'll be all-day units like the HD-600s are.
Sound quality is pretty good. They're not even close to the quality of my HD-600s, but they're not bad. Treble and bass are both a little constrained compared to the 600s. They don't have the air and power that the dedicated headphones do. (the HD600s are my absolute favorite listening device.) But imaging is very good, and for gaming audio, they do an outstanding job.
Keep in mind that I'm comparing a $250 retail headset, with microphone, against a $520 retail pair of headphones, so this is hardly fair. ($120 versus $330 if you compare actual purchase prices.) It's just what I have available to easily compare with. I'd MUCH rather use the HD600s for real music, but as headsets go, the PC-350 is pretty good. I like the isolation and the imaging, and while I could tell the two apart 10 times in 10 if blind testing was even possible on headsets, for $120, the PC-350s aren't bad at all. And they beat the absolute snot out of the surplus Plantronics unit I got for $10.
As a side note, I'm doing as close as I can to pure A/B testing. I'm driving both units simultaneously off my Total Bithead (which has two headphone jacks), and have them as close to level-matched as possible. I'm swapping ears back and forth, so when I say that they're missing quite a bit of bass and treble compared to the audiophile set, that's a pretty solid observation.
I'm not familiar with other headphones in that price range, so I don't know how they compare. But I definitely wouldn't buy them for music.