Get it right, though, and the fit is very comfortable, with soft padded earcups and padding at the top of the headband. Though most headphones need bending to fit, I found that these sat perfectly without alteration.
The sound is very balanced, but a little on the exciting side of neutral. Unlike Beats, there isn't a bass bump; instead there is a presence bump and quite a significant one. This type of bias is common for high-end headphones designed to be listened to at home, but it doesn't make much sense for listening on the move. The reason for this? If you're on a bus, train, or plane, there is usually so much rumble as to drown out any bass, so you'll want a little bit of a boost to even the response out. Even though the cans will go deep, the bass on the K551 headphones is still fairly slender, even in a quiet room and just gets obliterated on the subway.
The best thing about the K551s is that there isn't a "stuck inside my head" feeling. While you're not ever tricked into thinking you're wearing open headphones, the sound stage is nice and wide, and there is the elusive sense of "air" that many audiophiles crave. I only wish that the headphones were able to give rock or dance music the kick in the butt that they need.
For example, "Salt in the Wounds" by Pendulum is meant to be a bass-heavy assault on the auditory nerves, but in the hands of the K551s, it verges on a sibilant screech. There is some very deep bass in there, but it is so overwhelmed by the headphones' overly forward treble as to be almost inaudible. Rather than being pummeled with wub-wubs as expected, it sounded like I was listening to next-door's dance party over a fence.
If you like chamber music or anything with a sense of acoustic space, the K551s do this very effectively. Get out your harpsichord and have a Bach party, whydontcha? Tracks with tighter bass response -- like Talking Heads, or soul, for example -- also work surprisingly well.
AKG is going for the mobile audiophile with the K551s, a market that hasn't really existed -- except for a passionate few individuals -- until very recently. While Sennheiser's Momentum also fits into this category, it was seemingly designed as mobile from the start. "Mobile" feels tacked on with the K551s. For audiophile sound on the go at this price, I'd instead recommend something like the Bowers and Wilkins P5s -- they're compact, sound more even-handed, and don't experience bass suckout when things get noisy.
The AKG K551s look serious and they're well-built, but I would buy the slightly cheaper K550s without the mic and leave the headphones at home.