Back in January it was announced by Blizzard that they had reached over 10 million subscribers. That's a huge milestone, as there are plenty of countries with a smaller population. However, Valve scoffs at those numbers since they just hit 15 million.
Sure, Blizzard charges its users $15 per month and Steam is a free service, so Blizzard gets more steady income from their service. Personally, I'm a huge fan of the service. I love the fact that I can just uninstall and re-download games on a whim. Granted, there is a bit of a wait when you download the games, but when you've got a 10MB down connection, it's really not that long.
Valve also noted that their sales via Steam were up 158% this past holiday season over the previous year. I'm sure when they release Portal as a stand-alone download, they'll see those numbers increase even more.