According to this article Spotify is doing quite well in the US so far.  Unverified sources say that it has over 1.4 million users, but the especially interesting (and very important!) statistic is that they already have over 175,000 paying subscribers in the US.  

That’s critical, because for a subscription model like theirs to succeed, a significant number of users have to convert to paying subscribers.  This means that those enjoying the free level of service will continue to be able to do so, and those who are paying customers can hope that the money coming in will encourage whatever holdouts there are to start making their music available via Spotify and other services too.

Plus, a high number of subscribers will definitely make Apple sit up and take notice, and that will make for interesting times!