maybe so, but I'm not a hardcore music person, all of my music are pop radio top 40 hits. I mean if I'm listening to them they've already made it. I do tried to still buy from itunes for like random youtube artists or small bands, but that's pretty damn rare.
You get ads, and you lose access to the mobile version. (The mobile version will let you play an artist radio like Pandora, you just can't pick which track you want.)
Why pay for Spotify? Free Spotify on the web player (Chrome) + Adblock Plus is great. As others have said, only drawback is no mobile streaming.