If you've come to this page then chances are you already know these headphones are pretty special. I just thought I'd add my views after owning these for a week. For comparison I also own Senn HD250ii linears (now discontinued), HD650s and Shure SE530 in ears. All of these are very high-quality, and expensive headphones, so the benchmark is set pretty high....
The HD25s 2 ii (these are NOT the SP version, which are slightly lower sound quality, slightly cheaper versions) are tough, and businesslike. They aren't "fashion" headphones (I'm looking at you Dr Dre), which means while they aren't quite as pretty as Dre's Monster Beats, they sound rather better. If you also happen to be considering buying the Dr Dre 'phones then, having tested them, I found that they had very big bass but just average sound quality (the Pro's are pretty good to be fair, but the bass is still over-emphasized to my ears). If what you're after is very boomy, but flabby basslines that rattle your teeth, and the hell with the rest of the music, then the Dre's might well be for you. But if you want to hear your tracks the way they are supposed to sound...then the HD25's are light-years ahead.
The bass on the HD25s is big enough to tickle your eyeballs if you crank it, but it's not over emphasized, so is always tight and well-defined, with a nice tone and great punch. The mids are lush and well presented (though there is a very slight hole somewhere in there to my ears), and the highs are smooth and sweet although occasionally I find myself wishing the high frequencies were pushed a bit further back like in the HD650s. I personally like a slightly rolled-off sound in headphones, as it's the high frequencies that can really damage your ears, and these things can get damn loud!! Despite this there are no problems with nasty sibilance with these cans, and the highs really sparkle well on most tracks. Be aware though that these cans can go VERY loud, if you want bleeding ears then they can certainly be cranked to painful volumes.
The sound isolation is good. Not as good as my in-ear phones in my view, but as good as it gets for the supra-aural design. In most circumstances, including on my commute on the the train, it's still all too easy to forget where you are and escape into the music.
The soundstage is pretty narrow, even for closed cans, but not terribly so...each instrument is still easy to follow from left to right. The separation between instruments is surprisingly distinct. The soundstage would really only be a problem if you plan on listening just to classical music, or very sparse edm; in those cases the Sennheiser HD650 area a way better option, with a fantastic open and spacious sound. But you can't wear HD650s outside the house, and they cost almost double what these do! But there are other options that might suit classical music slightly better...
The cable is steel, so it won't break (unlike the cable on my shure se530s). Some people like to change the cable for expensive copper ones. I don't personally believe it's worth the bother, even if it does marginally widen the soundstage. The cable on these is tough as nails, yet the sound is still much more than adequate. And tough means they're perfect for travelling. The components are all replaceable too...bonus.
I'm going to use these next time I DJ, and I also use them for some dance music production. As reference headphones go they are great, very clear and revealing, so a good choice for budding bedroom producers...especially for the more dance-oriented genres.
All in all, I'd highly recommend these to pretty much everyone. 5 stars.