There's quite a few fake previews on Youtube, but it seems like a good idea once you think about it.
Both Half-Life and HL2 are good, but I think Half-Life would make a better movie because:
1) Simpler plot
2) More violence/gore (sadly, some people look for that in movies)
3) The Half-Life series is awesome anyway
4) 3D special effects will almost certainly be necessary, and a select movie audience also looks for movies with realistic effects above the plot (sadly)
Cons:
1) Gordon's voice. Even if they decide to get a voice actor, some fans may not like how he sounds
2) A big part of the game is what you do yourself, so having actors/special effects do it for you may be tricky
But in general, if you get past the cons, it seems like a really good movie idea.
Also, Modern Warfare 2 seems like a good movie, considering the great plot and how war movies tend to do well. But no matter what, the movies will have to be altered in many ways, considering the plot is based off a plot where you do most of the things yourself.
Note: this is about a possible movie about 2 great videogames. Keep hate rants about HL2 and MW2 to another topic...