Half-Life has consistently distinguished itself from other video games, and first person shooters in particular by displaying a strength of quality and depth in its characteriztion, and storyline that preivously would have been thought reserved for other mediums of story telling. However a movie consisting of 8 hours straight of headshots and silent jumping puzzles would fail to capture the entertainment of half-life. thats because the whole strength of the story lies in the fact that YOU are Gordan. so not being able to interact, just having to watch would be torturous no matter how good the film was.
Plus Hollywood would entirely butcher it. I mean, look at Max Payne. that game had above average narrative and characteriztion. it was a highly cinematic film-noir style story. you would think it would lend itself perfectly to a film adaptation. but instead they completely dropped the story, the symbolism, the characters, and threw Marky Mark into a leather clad tide of ridiculous eye candy girls and completely idiotic villians. epic fail.
fan generated content is always better, because the fans actually are familliar with, and like the story. instead of Marky Mark, who i think i remember hearing had never played or even heard of the game. probably same for the director too.
As far as fan movies go, i think escape from City 17 got it right. the half-life universe presents lots of opprotunities for cool stories to be played out, that don’t have to be directly related to Freeman. there are a lot of possibilities relating to the resistance, the 7 hour war, the vortigaunts struggle against the combine. etc.