I've been using Spotify since the UK closed beta (a few years ago, now). At first it seemed like the most amazing thing ever, but slowly I fell out of love with it, and now I barely use it.
Why?
1) The ads. At first they were reasonably infrequent and unobtrusive. But gradually they became more frequent and more and more obtrusive. Eventually some companies were deliberately producing jarring ads to grab your attention.
Many of the ads were for music. Other music: music that I would never listen to and (more importantly) music that was totally incongruous and jarring to the music it was interrupting.
2) Disappearing Music. I created loads of playlists filled with interesting (often independent) new music. Gradually whole playlists disappeared and stopped working. It felt like my music was being stolen. To replace ownership with streaming (at least for the generation that owned CDs) the catalogue needs stability.
3) Missing Music. As usual it's the big, classic stuff. I bought a license for a christmas party at my parents house. The lack of Beatles, Stones, Queen etc caused a near riot. Spotify was booed off.
4) Skype. I don't know why (but I suspect its probably Adobe's fault ;-)) but Skype and Spotify on the PC seem to have a problem with each other. Maybe I'm seeing ghosts here but I frequently have problems with these two apps (and the Chrome Flash plugin). Eventually I had to stop using one or other.
5) The artist payments are miniscule-to-the-point-of-broken. See this:
http://www.informationisbeautiful.net/2010/how-much-do-music-artists-earn-online/
I realise that the first problem would have been easily fixed by buying Spotify Premium and I was going to do this until my favourite music started to disappear.
Hopefully, the US launch will be enough to persuade the small labels to return to Spotify (maybe they already have?) and with a larger advertising base better ads could be produced and more appropriate targeting of ads could make the advert-laden experience more palatable. Similarly, a US launch may convince the big rock dinosaurs to join. That, at least, would improve next xmas.
Don't get me wrong: Spotify is an amazing thing and has a ton of potential. The US (especially teen US and low-income US) is going to love it. The App is fantastic (kind'a magical when you first see how quickly and easily it plays music).
If you have mainstream, modern taste and can stomach the loud ads you'll get great service for free.
If you're a fussy indie-kid like me who mostly listens to whispering soft audio as you work, you may want to check how many of your favourite obscure artists are onboard before you buy premium.
As for artists - while Spotify is a great innovation and a valuable way to move young people from illegally torrenting to legally streaming music it needs to generate more money for the musicians for it to be truly sustainable.
Edit: To make clear this is a UK account (not a US beta) so the falling-out-of-love took a few years.