I saw and tested one of these at CES earlier this year. I was seriously impressed, the difference was huge (and I'm picky about my audio!).

They fill in the gaps caused by compression, especially in the extreme high and low ranges (where the mp3 format is notoriously weak). Think feeding a jpeg through a filter to remove the compression artifacts. Essentially its a preamplifier with built-in fuzzy logic . . . . well worth the price if you're serious about audio quality.