I had a chance to try out HD25 not too long ago...
They're surprising comfortable, despite the fact that they're circumaural and has a pretty tight grip on your head. They manage to stay on comfortably, providing very good isolation and still stay stable on your head during movements. So if you're looking for a portable, full-size headphone, it's hard to beat the physical design of the HD25 from that aspect.
Sound though.. is a different story. HD25 has one of the strongest bass I've heard from any headphone. Short of comparing it to DT770, since I never auditioned one of those, it has more bass than A900, CD3000, HFI-700, V6... It is indeed fun and enjoyable, but could be overpowering as well. It is beneficial in a commute environment, such as trains, buses, but definitely fatiguing for listening strictly at home/office.
There's something distinctively "missing" in its midrange, not that it's particularly recessed, but you just get a feeling that some notes aren't being resolved at all.. just "missing" entirely. In comparison with A900, it just doesn't feel quite as detailed.
The highs are a little weird as well, it's inconsistent from recording to recording. In some songs I tested, the treble seems to be light and missing, in others it is harsh and sibilant. Overall it is still acceptible, but definitely not up to par with the type of balance and consistency that a headphone of its price should be.
Lastly, soundstage on the HD25 is entirely stereo. There's basically no frontal imaging whatsoever. Sound is distinctively coming from right and left side, with a pretty clear gap of missing sound in front of your head. Another factor that could contribute to listening fatigue as well.
Basically, what it comes down to.. is that HD25 is an adequate trade-off in sound quality versus versatility if you want a closed, isolating headphone that's light and stays on your head with no fuss. Obviously A900 is not a good "portable" headphone, but HD25 is. However in sound quality comparison alone, I don't think HD25 has the type of balance and overall performance that should be apparent in a headphone of its price range. If what you need is portability and isolation though, it's hard to find a better ergonomic + sound combination than that of the HD25.