These actually sound quite decent for the money, delivering good detail and bass, with relatively well-balanced sound and nice presence and immediacy (translation: the music doesn't sound like it's coming at you through a screen door).
The bass holds together pretty well at higher volumes and manages to be more punchy than bloated. No, you can't expect too much from $20 headphones, but these measure up to a lot cans that cost $40 to $60, and I was able to wear them for more than an hour without experiencing a case of listening fatigue. That's saying a lot.
As for the built-in microphone, I was able to make a few calls without hearing any complaints from callers about sound quality. The inline remote just has volume controls, a call answer/end button, and no transport controls (skip track forward/back), but most people won't miss that feature and will just be happy that they can take calls while wearing the headphones.
As you can see, I have some misgivings about the Panasonic RP-HX40s' design. On the plus side, they are lightweight, look decent enough, and fold flat. They get a middling score for build quality and comfort, which means they pass, but not with any distinction. Or to put it another way: depending on your head and hair type, some will think they're pretty comfortable while others will be bothered by their fit (that said, few people will really hate their fit).
I'm more bullish on the RP-HX40s' sound quality and certainly appreciate the built-in microphone. As I said, these sound impressive for $20 headphones. So, despite a few reservations, I can recommend them, particularly to people looking to make the most out of limited headphone budgets.