I love Spotify. I am a major music freak but I get bored with stuff pretty quickly... this saves SO MUCH MONEY because you can change out music frequently, try new stuff, and make killer party playlists without spending a crapload of money on albums and tracks. Spotify has a much bigger library than pretty much any competitor (I can't say it does for sure, but I haven't found one with more music yet) and the UI is almost identical to iTunes, making it super easy to use.
I do wish it had some form of a radio option... that's Mog's one benefit over Spotify. Mog's UI is much more complicated to me and I love the extra 4 million songs Spotify offers. The best part is you don't have to go in and redo your whole iTunes library -- it also lets you make playlists and such with what you already own. Spotify is every bit worth the hype, all for the price of one album a month.