The service allows you to create playlists, add songs to your virtual collection, add songs to queue (which is a more radio-way to have them played). You can also export your iTunes library information to Rdio, so the system can automatically add these albums on your collection. Rdio is a social network too (allows Facebook/Twitter integration), and allows you to follow people, share playlists, create collaborative playlists.
The music quality is pretty good, ranging from 192kbps to 256 kbps, depending on the album. The Adobe AIR app is small and manageable on the desktop, not taking much space (although I'd have liked it to "glue" on the edges of the desktop automatically, and have its icon on the notification area rather than on the tasker). Its UI is slick, and up to the point. The cellphone apps are the same too. You can sync your collection to your mobile devices and take it offline if you like. The format delivered is obviously some scrabbled mp3 database, rather than plain mp3 files. If you like plain mp3 files to be able to copy in any device, you'd have to buy the album, which usually sells full retail price at $10. Not all albums are available for buying via Rdio though.
Personally, I'd say that the service is worth the money it costs. It gives you the ability to fully taste, by listening and re-listening music before you buy. The albums you like, but not enough to spend money on them, you can always listen via Rdio. The ones that you discover that you really like, you can buy via Rdio, Amazon, iTunes, CD. If that $5 or $10 that Rdio costs saves you $10 every month for that bad purchase you could have made, then it's totally worth it. Just like wine-tasting...
And if you are one of those people who never listen music at home, on their living room, via their HiFi system (or AppleTV, like I am), then you might never need to buy any album, and have your music craving completely fulfilled by Rdio's $10 all-you-can-eat offer.
Still, things could be better. Not all artists are on Rdio. Animal Collective's bulk of discography is not "live" for example. Currently there is no gapless playback (which kind of kills Pink Floyd)! There's no way to "star" songs (or import ratings from iTunes), which are important for the formation of "smart" playlists -- another must-have for my husband and myself. Other info, like year & genre, are also missing, info that's also important for the formation of smart playlists.
But these are just feature requests. Features that one day may arrive if enough people ask for them. Instead, I have a strategic advice to give Rdio: partner with Bandcamp, among others. The future of music is in bedroom artists. Stardom, and manufactured "artists" won't be as prominent anymore as the music industry crumbles. Instead, just like everyday people revolutionized photography via digicams/FlickR, the same thing will happen with video (especially with the latest crop of HD dSRLs and digicams), and of course, music. A lot of upcoming, successfully online, music is made by these artists that don't have representation on iTunes and Amazon. And that's a good opportunity for Rdio to grab this coming-strong cult, truly-indie, market. I guess what I'm saying is for Rdio to let unsigned/non-represented artists publish their music on their site easily, and if these artists want their music to be free for downloading, Rdio should allow just that without hassle. There's no reason why I need to jump from Rdio, to iTunes, to Amazon, to Bandcamp, to Jamendo all day long like a grasshopper, just so I can find all the artists I like. Bring music in one place. And if something is supposed to be free, let it be easily downloadable as plain mp3s.