Pros: Design, SQ, durability, case
Cons: None so far
The PX 100s are certainly a supra-aural can to be reckoned with. If they were about $20 cheaper, they'd beat all of the Koss offerings for best value in portable headphones.
The bass: thick and meaty without being muddy. Works as well with Jay-Z as it does for the LSO. Even though it is solid, it never feels congested. There's enough space to make it feel natural.
Mids: Just slightly behind the bass. Still nice and rich with good clarity. Guitars and male vocals sound quite natural without any sense of artificiality.
Treble: Very clear without sibilance. Very forgiving yet still quite detailed. Better than any other sub $100 portable can I've heard.
Soundstage: Best I've heard from a sub $100 portable. Whether coming from my DX50, iPod 5.5 or C3, the sense of space presented here is quite refreshing.
Imaging/Separation: Right on par with the soundstage. Certainly above average, although never to the point of seeming "fake" or "enhanced."
Overall, the PX100s are getting more head-time than any of my Grados save the 225s. They are worth every penny, especially if you can pick one up second-hand. The folding design makes them easy to carry in pocket and their light enough that they can be worn for hours at a time. Isolation is quite good, although not to the point where your life would be in danger when you cross the street. If there's a better portable solution for this price, I haven't found it. Certainly recommended!