I'm still burning my 25 in (and I do believe in burn in - partly because NwAvGuy says that he does, partly because I *thought* I noticed a difference with phones I burned in left connected to my PC when I picked them up a week later) but already I'm in love with them. 
 
They're often cited as being a superb rock phone and having a rather Grado-ish signature instead of the laid-back Sennheiser house style. I'm going to eschew words like "forward," "dark, aggressive" etc because they're meaningless, subjective, and subject to change by EQing. 
 
Instead:
 
- You can make the 25s sound bassy, mid-range dominated or trebleish by using your EQ. These things are very responsive to EQ!
 
- I literally heard things in tracks I'd never noticed before. Tracks I've listened to for years, often using my supposedly detail revealing Ety HF5s.
 
- For those who don't know, prAT! (I think I have the capitalization right - I didn't know the word yesterday myself, other than its English slang meaning) is a the ability handle sounds with lots of attack - ie sharp beginnings and ends - convincingly. So drums need prAT! big time, and human voices and acoustic guitar much less so. Electric guitar otoh needs it quite a bit - and so does piano. (Imo.) The 25s excel in prAT! Drums and electric guitar sound better than I've ever heard them before. And I've just switched to a Ben Folds album and the piano sound amazing.
 
 
- Soundstage is there, but it's ***tight***. You don't just feel that you're on stage instead of an auditorium seat (as people say of Grados) - you feel that the band has formed a tight cluster around you, and start wondering why can't smell Gillian Welch's breath and why you're not getting sprayed with sweat by the Replacements. You remember that story about John Cale beheading a (dead) chicken on stage and wonder if the blood will wash out of your clothes. The instruments separate out magnificently, and you have a feeling of intimacy with the performance. I love it - but you might not.
 
I didn't test with orchestral classical music - my classical tastes are more for piano and chamber music. If the Eroica is the sweet spot in your listening tastes, then I can imagine that the 25's presentation might seem rather bizarre.
 
- Vocals sound amazing. Ms Welch really does sound like she's standing beside me singing directly into my ear. From a range of about 5cm.
 
- I've heard people say that certain high end phones won't tolerate bad recordings and sound worse with them than cheap phones would. I have no idea if this is true, but to my ears it isn't the case at all with the 25. I have an awful bootleg concert recording of the Replacements and the 25s, in cooperation with the J3's EQ, let my ears cut through the fog and murkiness to hear hidden detail. The result was a recording that sounded better than my studio albums did yesterday - you can still hear a lot of the sonic flaws, but the good stuff dominates and that's what matters. 
 
- I bought the 25s looking for phones that would make music sound as exciting to me as my old Mission 70 speakers did (these things were real killers for their day...) They've done everything I hoped there, mostly by way of that prAT! thing, but they've also revealed so much I wasn't hearing before that a lot of music has changed for me - I can see myself buying a lot more jazz, for example. And some later Clash recordings have just opened up and revealed magnificent textures that I never suspected they had - stuff quite at odds with my assumptions about them.
 
- Comfort Mine are fresh and the ear clamping is a little hard - I'm feeling some real pinching at the top of my ears. I understand that it would be less if I swapped to the velour pads, and that the band will become a less aggressively springy in time. At the moment it is a small price to pay.
 
- A nice feature is that the band consists of two arms. You can adjust the gap between these from zero to lots - the idea is that the twin arm design will cradle your head keeping the phones on under all conditions - these things are designed for news pros and DJs.
 
- Useful hint picked up from the net: you're supposed to set these so that they're slightly loose and then tighten the fit by widening the headband split.If you do this they're much more comfortable - I can barely feel mine on my head now, although they've also had 12+ hours of wearing to break-in.  If you're a DJ or in a TV news team, then someone will tell you this, I suppose. But for everyone else Sennheiser  really should have included a manual...
 
- These things have a rep for toughness - news people and DJs use their gear hard and they survive it for years. And because they're meant for pros, all the parts are user replaceable - you can buy every component from Senn and swap out broken band or a driver for a fresh one.
 
!!!IMPORTANT WARNING!!! 
There's one spot on each side of the band where the cable running between the ear cups can get trapped and damaged if you completely close the two bands together. There's a notch to prevent this, but the cable might not stay in it. If you're going to repeated close and unclose the bands completely - which is pointless - then you will want to watch this or add a dab of tape or something.
 
 
- Sounded great on the Clip+ and J3. I *think* they sound better on the J3, but I didn't do an ABX test and really don't trust my own feelings on this sort of thing. There was no problem getting them loud enough on either player. I keep them at about 22-26 of the Cowon's 40 level volume control.
 
 
 
- Looks: Industrial in the functional rather than the designer sense. These things are designed to blend into the background of TV news and sports broadcasts.
 
But... the 25s are *very*  customizable. You can change pads and cables for coloured ones, and even paint the band and shells. Or you can buy some amazing pre-customized phones from http://www.customcans.co.uk/
 
The owner of Custom Cans posts on head-fi as jfunk, btw. They also sell coloured ear cushions and what have you - including from an ebay store. They're based in the UK, but I think they ship everywhere. They customize other several phones, but the 25 seems like the core of their business.
 
- I've avoided giving numerical scores but for those who must have them, choosing some widely known comparators: if I gave the HF5 a 5 for sound quality, then the EPH100s - which are widely being compared with some of the best IEMs around - would be a 6.5 to a 7 and my Superlux 668Bs a 6 or maybe a bit more. The 25s would be, oh, a 9.5. Maybe they'll be a 10 when they're burned in. But I'd be the last person to deny that a lot of what makes a good set of phones is subjective, so ymmv. 
 
***Postcript***
- The HD25-SP has different drivers and a simpler headband; I've no idea how close it sounds to the "real" 25
 
- The Standard edition comes with a (cheap, unpadded) bag and two pairs of cushions - pleather (fitted) and velour
 
- The Basic edition comes without the bag and velours
 
- The Adidas is a slightly prettified version of the standard edition.
 
If I  was buying again I'd go for the Basic and use the money saved to buy funkier velour cushions from Mr JFunk - he also sells what looks like a very nice bag. Or I might go crazy and buy one of JFunk's special editions.
 
Finally, as so often, UK buyers get an awful deal on the price of 25s compared to Americans. So if you're in the UK and making a trip to the US, you might put buying a pair of 25s on your to-do list. But also, as mean and obsessive about US-UK price differences as I can be, I am ***really*** glad that I bought these!
 
- The estimable Mr JFunk of Custom Cans has provided a guide to tweaking the 25s acoustics here:
 
http://www.head-fi.org/t/590995/some-diy-mods-for-the-for-the-sennheiser-hd25-1-hd25-13-hd25-c-hd25-hd25sp
 
..And a guide to painting them here:
 
http://www.head-fi.org/t/573845/heaphone-airbrushing-guide-with-lots-of-pics