Bumping with some user experience.
I've been using the K551 for about a week now, and I completely adore the sound. It has precisely the sound signature I've been looking for for home and portable use, staying on the bit bright, clean side but also being able to reach deep down when needed.
Much like how I've heard about the K550, the correct positioning of the headphones might take a small while before you manage to get a proper seal. In my opinion it doesn't affect the sound too much though. I'm kinda big-headed personally, so the pressure level is comfortable and it stays on my head securely enough. The size might be a little bit silly to look at in very portable use, but if you use this while, say, commuting and such it shouldn't pose as a problem really. A carrying case and some extra adapters of sorts would be nice but once you have a few headphones you kinda don't really need any more. All in all, I only manage to find one major design flaw in these headphones though:
The plug doesn't fit all the way in with quite a few of the protective cases for iPhones and iPods. This includes the official bumpers for iPhones. This lets the sound work fine enough, but the microphone and remote controls will be offline.
With the cable being pretty much the only new thing in the K551 over K550, I can't help but to wonder how the engineers didn't take this to account. The iDevices are (some might argue intentionally) fragile pieces of equipment, and using them without at least some sort of protection is basically asking for an excuse to buy yet another expensive piece of fashionable elecrtonics. I don't mind the cable being non-detachable really, but limiting the prime function of the new cable in a manner such as this seems like a pretty major design flaw in my books.
So if you're considering about buying these, make sure that you can actually plug the headphones all the way in.