I just picked up a pair of PX100-ii today. I've been missing the PX100, and I've been curious how the sequel improves on the first one. First impressions (comparison from memory)...
Sound: the bass is a tiny bit louder and kicks harder, mids a little more laid back but a bit cleaner, highs the same. Running a frequency sweep, I think the highs on the ii are maybe a bit less wobbly. On fast bass heavy songs, the bass tends to bleed a bit into the mids. Other than that though, it just adds some emphasis to the midbass. Soundstage feels maybe a tad wider, but I might be imagining it.
Build: a bit thicker, it just feels nicer in your hand. The one sided cable is also a nice change. Folds up just as tiny as before. I miss the hard case though.
Comfort: If the PX100 rated a 10 on Joker's scale, this one is a 9.5 because it's just a smidge heavier. The clamp feels a bit stronger, but as these are new I'm sure that will loosen up. Otherwise, it just melts away and you forget it's there.
Verdict: They had a winning formula and didn't mess with it. The build is much improved, and the sound is tuned slightly towards the bass which is what the major demographic listens to anyways (though to be honest the sound signature is similar enough that it might actually be identical and it's just the placebo effect talking). The price is up a bit, which puts it into competition with a lot of good cans, but I would not hesitate to recommend these to anyone. If you already have the PX100 though, no need to upgrade.