For me, Spotify is fine as a supplementary service, like to listen to an album before you decide to buy it. But as far as it being my main music app, eh..
I only use it on my computer, so the ads really don't bother me. If anything, having to click through an ad every time I tab back to Spotify is more bothersome. Also, the radio feature is kind of crappy. If I were to subscribe to anything, it would be Pandora.