Spotify is amazing. If you haven't heard of it it's a service which is basically a massive iTunes store with millions upon millions of mp3s from various artists. For the average audiophile or music listener it's a great way to expand your listening of other genres, but for musicians it's an invaluable tool.
As a musician, when you want to study or really learn a piece you can't learn off of just one recording. You need several different interpretations of the same piece to really figure out a range of styles that a conductor might take of the piece. The old way of doing this is to find recordings in a library or from other musicians, however with the internet and iTunes/other online services you could easily purchase several different recordings. However, with Spotify you can listen to several different recordings of the same piece and get several different interpretations for the piece. That being said there are some issues:
Most orchestras tend to release through their own label, so the quality of playing is usually lower. That being said, there are some recordings of major orchestras, but not as wide as their actual library.
I'm not sure if Spotify is partnered up with Naxos, so Naxos recordings might not be available either.
Fairly add intensive for the free version. Not enough to drive someone crazy, but enough to be annoying on occasion.
That being said, they still have some great recordings for free. They keep up to date with new releases for pop/metal bands, as well as having some famous musicians/recordings with their solo albums. One of the most surprising is having Ian Bostridge's albums, a famous tenor, to listen to. The range of music this service has is surprising. It has a number of albums for Asha Bohsle, Various Klezmer, Indie, Balkan, and others. However, it doesn't seem to have any K or J-pop albums.
While it is a great step in the right direction for music diversity, it isn't s complete collection. That being said, it will keep a listener busy for several years.