Phantoms the Movie is a decent adaption of the novel and works fine as a horror movie, I really think so. There are a number of good setpieces, especially the church scene and the big reveal towards the end. But, and this is a big one, the movie has two giant flaws that stops the movie from greatness.
1. The script. There is a shitload of really preposterous dialogue that feels more at home in a Syfy original and that whole thing about Afflecks character and the kid he shot is just stupid. I am gonna whine a little about the changes from the book like whole subplots missing but it's not really that big of a deal.
2. The actors. Well mostly Affleck and Rose McGowan. Affleck does a decent job but he was 26 at the time, looks like he's 18 and plays a sheriff that used to be an FBI agent. Just stupid. Rose McGowan at the tender age of 25 plays what is supposed to be a teen and this feels just wrong. Not to mention the fact that she cannot act at all. Peter O'Toole brings a certain class to the whole thing but he has some of the most atrocious parts of the dialogue, poor man.
But besides these two nagging points, Phantoms is actually a quite fun blob-movie with some gooey sfx. And we all know we cannot hate blob-movies. There is actually a law prohibiting such behaviour. There is, I swear.