Is Mirrors a bad movie? Well, yes and no. Overall, it kept me entertained for an hour and a half. It was pretty intense, and while it was not scary, there were a couple of scenes that were tremendously shocking. (Think of the jaw, if you've seen it.) The movie, directed by Alexandre Aja, was suspenseful enough, and Kiefer Sutherland was likeable enough as Ben Carson to make me wish him well and see him extricate himself from the grim predicament he is in.
On the other hand, Mirrors is not exactly a breath of fresh air, and most elements in the movie were not very original. This is not necessarily a bad thing, and Sam Raimi just proved that in Drag Me to Hell, but Aja is no Raimi, and the movie suffers from it. Some of the special effects are quite bad, which really surprised me. There is a scene in which Kiefer Sutherland is apparently engulfed by flames, and I swear the fire in Gears of War 2 looks more real than that. But the worst offense, in my opinion, is the fact that Mirrors vaguely sets a series of rules for the world the story takes place in, and then violates them when it's convenient to move the story from point A to point B. In other words, the internal logic of the movie only holds true when it's convenient, and it is cast aside when there's need for something flashy or unexpected. You could argue that maybe I missed something, but I guarantee you I did not. I watched the movie with a couple of friends, and I asked them a bunch of questions regarding particular events and situations, and it took the three of us to kind of come with some semblance of a theory to somehow explain some of the developments of the film. The reason why we were having such a hard time wasn't because we're dumb, but because sometimes things happened without rhyme or reason: the plot demanded it, and it had to happen-- simple as that. Unfortunately, that's poor writing and lazy storytelling, and that is the thing that bothered me the most.
To sum it up, I would recommend that, unless you want to see Amy Smart's butt (not too unpleasant a prospect), go ahead and skip Mirrors, unless you are a scary movie buff. If you are, I think you will be entertained and little else. If you're not, though, this movie will probably come back to haunt you, and for all the wrong reasons.