Even though Spotify is frequently compared to Pandora, they’re not the same type of service. Pandora is a radio, whereas Spotify is more like iTunes. In Pandora, you don’t choose which specific songs you want to hear; in Spotify, you do. Spotify is basically what iTunes would be like if, instead of paying 99 cents or whatever per song, you paid a flat monthly fee and then listened to the songs in the iTunes player for however many times you want. You have to pay extra to get Spotify offline, on your mobile device, or be able to download songs. Spotify is a lot more like Last.fm than Pandora.
While Spotify does technically have a free option, it sucks so hard that it’s basically worthless except to see what Spotify can do; there are tons of ads you can’t mute, and you’re limited to a few plays per song and a few hours (5, I believe) of total listening per month. The paid options are 4.99 (US dollars/pounds/AU dollars) for no ads and unlimited listening and 9.99 (again, US dollars/pounds/AU dollars. Yes, it costs more for Americans than for Brits or Aussies because of the exchange rate) for the premium features (ie offline access, downloads to your iPod).
There’s been some criticism of Spotify. It’s hard to unsubscribe, there are bugs in the various mobile apps that have led many to unsubscribe from the premium features, and that they don’t pay artists very fairly (for example, for 1 million plays of “Poker Face”, Lady Gaga earned $167).