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 them at home.
Read the full review of the AKG K 551.