Just got a pair of these. I like them. Nothing fancy, but they get the job done. The K550, IMO, is a better headphone, but you can't take those around with you without looking silly.
I like these better than the H6, the PM-3 (I bought and sold\returned both recently) and a bunch of other portables which I've owned or heard over the years. The H6 and the PM-3 are better headphones (technically), but somehow I prefer the signature of the K545.
I bought the HD280 headband...and I might try the HM5 pads. The headband bothers me...which is strange since I had no problems with the K550s. The pads could be a little roomier -- I hope the HM5 pads don't change the signature too much. I guess I can live with the stock pads.
Just some random thoughts...
@elvergun put some brainwavz pads on them (AKG545_ and you will change your opinin about the K550. imho