Summary: For closed back headphones the AKG K551 is surprisingly open and airy. The precision and clarity on the K551 is through the roof. The AKG K551 sticks to its reference monitoring legacy to the tee. However, if you want your 20 to 150 KHz space bumped up, these are not the cans for you. If you want analytical sound, and don't mind the bulk, look no further.
Pros: Surprisingly open and airy for closed back cans, Great clarity, Eye catching design
Cons: Big and chunky- not really portable, Folds flat, but not further
Summary: The AKG K551s are closed portable headphones with an open, airy sound, but their relative lack of bass means they're actually better for use at home.
Pros: The AKG K551 headphones have a sturdy build and are very comfortable; closed headphones offer some passive noise reduction; airy sound suited to classical or acoustic music.
Cons: Loss of bass in noisy environments even with passive noise reduction; treble can overwhelm bass with some music; not suited to rock or dance; obnoxious-looking; no carrying case; large and not as portable as alternatives.