Pros: Light, portable, good sound quality for the price
Cons: Lack some sparkle at the top end, no isolation whatsoever
I bought these because they were cheap. Yes, $60 is cheap in Korea for entry-level audiophile cans, due to horrendous import taxes. I chose the PX-100s after a good hour of A-B comparisons in-store with their rivals, principally models from Koss and Sony.
More than two years later, other headphones/IEMs have come and gone, but I've kept the PX-100s because they are just so damn handy, and they really do sound good for what they are. It's nice being able to fold them up into their small carry-case and toss them in a shoulder bag where they are so unobtrusive you can almost forget you're carrying them.
Their strengths lie in their light weight, class-leading portability and sound quality (for the price). Their negatives include the open-backed design, which lets in all sorts of horrible noises on public transport, and the fact that their lower-end warmth comes at the expense of higher-end sparkle. One "fix" for the lack of isolation would be to purchase the closed-back PX-200s. Unfortunately, their sound quality does not match the PX-100s (note that Sennheiser has recently introduced the PX-100 II and PX-200 II, which are slightly different beasts, and I have not heard those models).
More expensive Sennies such as my girlfriend's HD238s and my own HD25-1 IIs do have better clarity and are less coloured than the PX-100s. However, the PX-100s are better than a lot of IEMs that cost twice or three times as much. For example, in an A-B comparison with my $200 Future Sonics Atrios, the PX-100s have much smoother mids and flatter highs and offered a generally much more pleasant listening experience.
They're not the best cans in the world - you'll have to pay 10 times as much to get those - but for the price the PX-100s are definitely five-star winners.