Brad Wardell, Stardock CEO, recently stated that Steam is leaps and bounds ahead of competing digital distribution services -- including "Impulse", Stardock's own gaming content delivery service. In terms of dollars generated per month, Wardell estimates that Steam enjoys some 70% of the digital distribution market, while Impulse makes up about 10%. All other services combined account for the remaining 20%.
Wardell also noted that Valve is beginning to monopolize on its position by licensing Steamworks to major publishers as a DRM solution. If a game requires Steamworks, it is essentially cut off from Impulse and other services, thus limiting their content. Examples of this include THQ's Dawn of War II, Sega's Empire: Total War, and more recently Activision's Modern Warfare 2. Wardell says that it is impractical for his company to provide a game that requires the installation of a competitor's store and platform in order to play it.
Stardock is working to evolve Impulse by improving the user and community experience, in addition to reeling in more content.