I'd tried Pandora briefly a number of years ago. I dimly remember being unimpressed at that time. Summary: either Pandora's gotten a lot better, or my standards have gotten a lot lower. I was prompted to retry by a recent Steven Levitt post at the Freakonomics blog.
Briefly, Pandora is an Internet "radio station" that allows you (once you've established an account) to input some of your favorite artists or songs; the player then uses software based on the Music Genome Project to provide a playlist consisting of songs that resemble your favorites' musical "DNA."
For example (as I type), Pandora's playing "So Far Away" by David Gilmour. Hey, it's pretty good! When I ask Pandora why she's playing the song, she replies:
Based on what you've told us so far, we're playing this track because it features mellow rock instrumentation, acoustic rhythm piano, major key tonality, and many other similarities identified in the music genome project.(Apparently I'm a huge fan of major key tonality. I see that a lot when I ask Pandora for explanations.)
As songs play, you can hit "thumbs up" or "thumbs down"; this allows Pandora to (if I'm reading their FAQ correctly) fine-tune your preferences. So—in theory—you can tell Pandora that you like The Who; judicious use of the thumbs might let you refine that to Tommy/Quadrophenia/Who's Next-era Who instead of "early" Who or "dead-Moon" Who.
After listening for a few days, I'm impressed. Pandora plays good stuff by numerous artists, some of them I've told Pandora about, and some I've never heard of. And it's particularly impressive when Pandora "deduces" one of your favorite artists that you haven't told her about. She's pretty and smart!
As near as I can tell, however, Pandora doesn't make any judgments based on song lyrics. At least, that's the simplest explanation for her offering Harry Chapin's "Cat's in the Cradle". That might be listenable—if Harry had ripped out the mawkish lyrics and sung something else instead. But as it is, approximately two seconds into the song: Emergency Thumbs Down! Down, I say!
Similarly, Pandora played a Reba McEntire song, which was entirely fine—until Reba stopped singing, and started talking. You know, the way country singers sometimes do. Sorry, Reba; the only musician I allow to yak at me is Van Morrison. Thumbs Down!
As you can tell, playing with Pandora can be a little bit of fun.
I'm also hoping it will help detect some new artists for me. I used to rely on FM radio ("The River - 92.5 FM") for this, but ever since I got a car player for my iPod, I don't do that any more.
The basic service is free, supported by unobtrusive advertisements on the web-page player. (You can buy a subscription to be free of ads, which also allows playback on some cell phones and home audio gear.) There are "social networking" features for those people who like to share.
[Update (2007-09-30): I should also mention that, even though Pandora only advertises support for Windows and MacOS, it wasn't too tough to make it play nicely with my home Linux box, running Fedora 7. Although, as installed, Fedora doesn't understand the license-encumbered MP3 streaming format, or contain a Flash player, you can follow E-Z instructions here to remedy those shortcomings.]