Pros: Neutral, detailed, non fatiguing, comfortable velour earpads, cable can be replaced with sennheiser HD650 cable
Cons: Not much soundstage
This is my reference for what a portable closed headphone should be.
It's neutral and sounds good on all genres.
It's not a giant killer like the DT531, but keep in mind it is closed and portable and has the most durable build quality of anything I've seen.
Also, it has the cheapest, most cost effective upgrade available for it, the upgrade to an HD650 cable.
In my opinion it is the second best deal to be had in headphones for a beginner to this hobby, second only to the Beyerdynamic DT531, if you can find it.
The Beyerdynamic DT531 has better high range sparkle and soundstage, but less midrange realism, depending on the genre you're listening to.
Anyways, neither are a perfect headphone, and calling either a giant killer is an exaggeration.
What I mean by that is for someone new to the hobby, the Beyerdynamic DT531 and Senn HD25-1 are just an amazing deal in terms of price: performance ratio.
Yes they are bested by even mid-fi headphones in the $600 range. But considering the price you can find these for used, I'm not sure I'd recommend someone to 'upgrade' and spend 500$+ an amp if they're happy with the sound.
Listening to John Coltrane - A Love Supreme I preferred the HD25-1 to my HD650 balanced system powered by a Beta 22/Lavry DA10.
The HD25-1 has a seriously sweet midrange for being as neutral as it is. Tonally accurate, yet easily heard in the music, almost 'highlighted'.
This mysterious midrange quality is it's greatest strength and the most stand-out part of the headphone followed immediately by it's tight, tuneful base.
Now for the negative part. Over time the closed in soundstage makes them less fun to listen to. It kind of builds on you over time and threatens to distract you completely from the listening experience.
But if you listen while doing other things, like snowboarding or skateboarding, biking, you don't notice this.
That's all I really have to say for negatives. For the right person, these are the ultimate portable and rugged, yet amazing sounding headphones.
If they redesigned the arc assembly so that the headphones folded outward when sitting on your neck, we'd have a new FOTM (flavour of the month) on head-fi.