I saw it last night. I agree in that the premise was pretty cool.
Other than that, I thought it had a lot of flaws and I have this sneaking suspicion that its rave reviews are due in part to the perpetual hard-on that film critics seem to have for foreign films.
The scenes, as well as the plot, were disjointed. Certain scenes weren't properly created or edited to maximize their scary or suspenseful potential (for instance, the part where they're changing the tire in the tunnel).
The Brits have a weird since of timing in their films, particularly as regards humor and comic relief. There were a couple of times where it just wasn't appropriate and detracted from the sinister tone of the scene and/or act.
Some parts should have been left on the cutting room floor, like the part where they're in the tunnel, and Frank decides to drive up over the car pileup. Suddenly they're virtually flying through the air in that taxi with goofy little smiles on their faces, and you get the feeling that you're watching a scene out of "Harry Potter".
The character development of Selena and the attempt to show her transformation from a tough, dog-eat-dog survivor to a softy-at-heart seemed very clumsy to me. Normally, you don't worry about character development too much in a scary movie, but that's because it's generally not expected and it's accepted that that is not a priority in the creators' mind. But since this movie was obviously trying to delve a little deeper and be somewhat more artsy, it bears mentioning.
Overall, I'd say it was a decent movie but very much over-hyped. I have the feeling that a lot of the potential of the premise and setting was squandered.