While there remains some skepticism from cynical industry pundits about Spotify and its delayed entry to the US, feedback from all its users I know has been unhesitatingly enthusiastic, and paying for the service has been a no-brainer. I'm currently signed up to pay $9.99 a month automatically from PayPal to Spotify, and have considered closing other paid music accounts, including Sirius XM and the competitive Rdio, who coincidentally introduced its Mac OS X application earlier today.
Daniel Ek, founder of Spotify, commemorated the milestone today in a quick blog post, saying "It’s a testament to our fantastic users who continue to support us and spread the Spotify word." Early access to albums like R.E.M.'s "Collapse Into Now" doesn't hurt either.
Recent news has discussed Spotify's ongoing discussions with US record labels about entering the states, negotiations which don't sound fun, fast, or cheap, but are clearly happening. It's also probable that Apple's controversial subscription battle was designed with Spotify in mind, though they have opted to stay out of that particular PR thicket.