Pros: Comfortable, spacious open sound, great build quality
Cons: Very sensitive to seal, overall lean sound, somewhat raw treble, slightly heavy
Along with the summary below, I have posted a Youtube video review of the K550. If you like the video, check out my channel for more reviews :)
The AKG K550 has excellent build quality, with a great fit and finish and a solid, durable feel to the materials. The earpads are very large and may not fit everybody's head; despite being very comfortable and supple the earpads are sensitive to seal and small changes to positioning will affect the already quite lean bass. The headband padding is quite sparse and because they are quite heavy, after a few hours they may make the top of your head a little sore.
The AKG K550 has a very lean, dry sound. What is most noticeable is the spacious soundstaging and clean sound despite the closed design. The K550 has no real sense of congestion or anything characteristic of closed headphones. However the sound is very lean; bass extends very deep but there isn't much slam or warmth. The treble can be slightly raw on occasion. Overall they are balanced headphones good for a variety of music but on the leaner side of neutral and not to my personal preference. Fans of more analytical signatures may enjoy these.
These are well made headphones with a good sound, but the fit issues mean that you may need to trial these before buying them. The lean sound may also not please everyone, though I commend AKG for designing a closed headphone that sounds very much like an open one.