Showing up on various forums, the .cab and service is actually very, very polished. It is the official version, but SlackerRadio themselves have not published it on their site yet.
No compromises here as you get album images, lyrics, band bio, reviews and a wide selection of music to choose from. Audio quality is also extremely good, trouncing Pandora's 64kb stream. Plus they got that whole "let me power off the screen without disrupting the stream" thing.
Two negative's we've noticed: landscape support is not there and we've had a few songs cut out towards the end for some reason (not uncommon with Pandora either).
Of course, there is the free "basic" version which limits you to something like 6-skips per hour while the program steers you towards the "full" version with upgrade notices in parts. Likewise, you don't get all those lyric bonuses in the free version.
If you want the subscription, it's $4.99 per month (on a month-to-month basis) or $3.99 a month on a 12-month plan (billed monthly). You can also try out the subscription for 7 days for free. We suppose if streaming is your thing it's not a bad deal: the quality of the product is quite top notch. If you just like streaming your own music (as opposed to the jukebox, random streaming), give nanoGroove a shot.
Edit: Whoops! And just as fast Slacker Radio has remotely pulled the plug. The app currently doesn't work and we have to wait for the fine folks at Slacker Radio to drop this app "officially". Sigh. [Here's their official reasoning]