The first three-quarters of Orphan is a creepy, albeit insanely dumb, horror movie. It's got a lot of queasy-making moments (while it's not Saw-level gross, they could have easily titled the movie Children in Graphic Peril, and a really uncomfortable moment near the end will cause any reasonable adult to squirm in disgust), and they got a cast full of brilliant actors to polish this turd. There's Vera Farmiga playing a quite-unlikable mom; Peter Sarsgaard, awash in yuppie smugness; the always-delightful CCH Pounder as an expository nun; and Isabelle Fuhrman is phenomenal as the titular orphan. Sure, the plot explodes as soon as you apply any miniscule amount of common sense to it, but the cinematography, direction, and performances are more than just unbad; they're really quite good.
And, yes, the twist is everything I had hoped it would be. The last quarter of the movie ups the dumbness factor to insane levels. But really: Nearly every horror movie drops the ball in the last reel. Let the Right One In is a rare exception, and the original Texas Chainsaw Massacre is, too. And Joshua (a movie that could be Orphan's fraternal twin, and one which also stars Farmiga, who is perhaps publicly working through some children issues) already took the best possible ending that an Orphan-like movie could wind up with. So I can appreciate the filmmaker's decision to just totally go over to the deep end and embrace the idiocy. It does what mainstream horror movies are supposed to do, in that it soothes the audience after riling it up for an hour and forty-five minutes. That Orphan chooses to calm the audience down with laughter by twisting itself into a hilarious pretzel of a plot twist would be genius if it weren't so dumb, and it makes Orphan a fine leading-up-to-Halloween horror movie.