The problem with the Splinter Cell movie is Michael Ironsides IS Sam Fisher, he just is. No two ways about it. That voice is what makes Sam, well, Sam.
Unfortunately, he is too old and looks nothing like Sam Fisher. Which means the Splinter Cell movie is already starting on a bad foot because they are going to be missing the most defining point of Sam, his voice.
As far as the Halo movie goes? I'm going to reserve judgement until more is known before I condemn of exault it. After the Lord of the Rings' movies I trust him. I think he might be able to pull it off, especially because he holds enough klout in Hollywood to get the budget you would need for a movie like this.(Though he does seem to have a strange fixation with New Zealand).
Mind you when it is revealed that the Rock is going to be playing Master Chief.
All is truly lost.