My turn! In the category of Most Overrated...
3rd place goes to the JVC Marshmallows. They tend to get far too many recommendations in the cheap earbud-replacement category. At least for me, they're painfully uncomfortable, and take some coaxing and patience to get a seal with, and even when I get a seal they really have mediocre sound at best. Maybe I simply had a bad pair, but I just couldn't bring myself to like them. And if I did have a bad pair, then that tells me that there's an issue with quality control (no surprise, I suppose). IMO, Marshmallows are outclassed by the Sansa Clip's stock earbuds.
Second most overrated is... drumroll please... Bose. (You had to know they'd be on the list somewhere.)
Which brings us to the absolute most overrated: Skullcandy. Skullcandy is like a parody of Bose, minus the illusion of class. And boy are they horrible! From the two models I've had the displeasure of listening to, they have incredibly bloated, muddy, one-dimensional bass, and recessed treble, plus essentially no detail anywhere in the audio spectrum! If you haven't heard them (lucky you!), to give you an idea, it sounds a little like someone customized their car with an overly expensive sound system and spent most of the money on a set of overpowered subwoofers---and you're listening to whatever leaks out through the car's metal chassis, from across the street. It takes as much EQ as most MP3 players can handle just to suppress the bass enough so that you can start to hear a little detail. And even then, only a little. And to top it all off, they can be pretty overpriced. BTW, it doesn't help that you'd have to pay me to wear them in public, what with the garish skull logo and flashy, obnoxious casings. My one piece of praise, if I had to give one, would be that Skullcandy tends to make fairly comfy 'phones. But who cares, when you can't stand to wear them anyway, for any number of reasons!
As for underrated, I'd say...
Bose. ...And you're thinking, "WHAT?! You can't have it both ways!!!
How can they be overrated and underrated at the same time?!"
Well, let me explain!
They're overrated by reviewers and consumers at large. [Now you're furiously nodding agreement.] At the same time though, they get a lot of undue hate around here. They're not as bad as they're often made out to be. In fact, they're not bad at all... at least, not for most people. The problem of course, is that they're so overpriced, it's almost impossible not to look at it as them versus the more refined, more respected competition in their price range, in which case they often lose, sometimes pitifully. That doesn't make them bad products, it just makes them grossly overpriced.