Steam is pretty much the de facto standard for PC gaming these days. Even the most retail-loyal PC gamers likely have used it in some way, shape or form. Valve's Steam-powered empire also dominates the digital distribution landscape. Brad Wardell of Stardock (which owns the Impulse digital platform) estimates that Steam has roughly 70% of market share.
But it's one thing to be the big kid in the playground. It's another to start locking the other kids out. PC gamers may recall a brief dustup over the sale of Modern Warfare 2, wherein Direct2Drive, Impulse and other digital vendors refused to stock the title due to its native use of Steamworks, which would require the installation of the Steam client, and thus the Steam storefront.
I can see things from their point of view, certainly. Why sell a game that, once installed, provides a back door for the competition? Valve has also been lobbying hard for third-party publishers (like Activision and EA) to start including Steam's native DRM solution, which again could shut out competing services (like Impulse's Goo DRM).
Then again, the ever-outspoken Derek Smart published a very detailed counterargument on Gamasutra as to why the boycott itself was rather foolish, essentially making Modern Warfare 2 a Steam-exclusive title. The "problem," Smart claimed, was that Steamworks, as a unified suite of services for multiplayer, auto-patching, storefronts, and authentication, offers much more value to the publisher and developer. Essentially, Steam rules because it's simply better. Steam boss Jason Holtman showed the same sentiment on Gamesindustry.biz:
To our minds, we think that if you're making a good game and it's got the services a customer wants it should get out in as many channels as possible. If you have a good portal and you're good at collecting money from folks, and attracting them, there's no reason why you shouldn't be.
And I think he's right, in that sense. I choose Steam because it's awesome, and the closest PC gaming has ever come to a platform like PSN or Xbox Live, with the added advantage of there actually being competition. While the potential for abuse is there (as Randy Pitchford complains), there's always an alternative. Should Valve start to throw its weight around too much, I might just choose Impulse, since it has Sins of a Solar Empire on it. Just, for God's sake, don't sell it on Games for Windows Live. Please.