A Portal movie would be funny, if only to hear all of the people saying "Hey, that robot in Portal stole the voice from Pacific Rim!".
Personally I think game to films should only be done if they are as faithful as possible, or if they are a part of the extended universe and do not follow the story of the game. Mass Effect, Halo, Dragon Age and other games could be made into decent movies if enough effort was put in, but games such as Half Life and Portal don't really have a whole lot going on outside of the games story, at least not to the extent that a decent film plot could be created.
Personally I think game to films should only be done if they are as faithful as possible, or if they are a part of the extended universe and do not follow the story of the game. Mass Effect, Halo, Dragon Age and other games could be made into decent movies if enough effort was put in, but games such as Half Life and Portal don't really have a whole lot going on outside of the games story, at least not to the extent that a decent film plot could be created.