I've read the books and although the casting may seem shocking I'm going to reserve my judgement until I see the film. I remember people having absolute fits when Reese Witherspoon was set to play June Carter Cash. She went on to win an Oscar and give an absolutely fabulous performance.
I think some people are inclined to comment on Cruise more because of his lifestyle choices. The last Mission Impossible movie was quite good and he at least does most ,if not all , of his own stunts.
I think if we can suspend our beliefs to allow the wonderful but extremely old Liam Niesson and Harrison Ford to explore action hero roles then why not Tom Cruise?