I just got my pk2's! Initial impressions: great soundstage, great detail, and oh so portable.
These easily just replaced my px200's. When comparing them side by side, the yuin is so much more lively. It really brings you into the music; an impressive attribute for and earbud.
CX300's are very close; very different, but very close. I feel that the cx300 has too much bass and not as much detail. While it does have some isolation, thats not what I was looking for in these 'buds. While the cx300's may be the better choice to use on the bus, I will be carrying the pk2's much more often.
Lastly, for anyone interested in purchasing these, you may want to wait for the "y" cord version. I really don't mind the "j" cord, but if you do, then there is always the second option coming. Oh, also, people have been complaining about the lack of bass. I feel that it has a fair amount. Many headphones just overdo the bass, and I actually feel that the amount that these produce is more lifelike.
Edit: I just listened to some jazz... this would be the area that these buds excel. AWESOME!! If you are looking for a pair of portable cans for jazz, these are them.