I wanted some highly portable headphones that aren't ear-canal fits (getting sick of headaches from my ER4s), so I decided to give these Audio Technica clip-ons a try:
I rather like them. The soundstage is really broad, considering the fact that they sit so close to your ear, and the clarity is top-notch - dare I say it matches the ER4s. They're a bit pricey, but perform well enough that I'm overall very satisfied. They're small and comfortable, and the design is a lot better than the other clip-on headphones, especially the (nice sounding if a bit muddy) Koss ones that are basically throw-away plastic/foam affairs.
The negatives would be a fairly weak bass, compared to full-sized cans, and some noticeable sound leakage (both in and out, though leak out is not so bad). Oh and they're REALLY tinny when you first put em on. I've never been a believer in burn-in phases, but I think I am after using these guys. I left em in the sock drawer playing music for a couple days and now they sound significantly more balanced.