My first and foremost thought about this story is that it is way too long. Given that this is the first book of a trilogy it may well be that some of the detail you have to plough through as a reader, may still have some point but whether anyone will remember any of this by the time the second installment is in their hands is doubtful at best.
I expect that by now any one who reads is aware that The Passage is a post apocalypse, vampiric, zombiesque, horror story of epic proportions. That said, I should probably tackle the white elephant in the room, which of course is how well it holds up against King's "The Stand". I thoroughly enjoyed "The Stand" and in fact read it twice over the course of a decade, so for me, at least " The Passage pales in comparison.
Bottom line.......the premise of the story is good and at least some of the characters are so well wrought as to make them memorable enough to survive, with an absolute plethora of unanswered questions. One can only hope that Cronin's editor sharpens their pencils well for the next installment.