I got to see this on Wednesday last week when it finally made it on UK general release.
The plot, is of course, sheer lunacy, but like Mars Attacks, that's the entire point. And as with all "B" movies, I've seen better scripting and acting (and again, this is what you should expect). Unlike many B movies, however, the overall production quality was, IMHO, very polished. I've read a lot from professional critics lamenting that Hollywood would have made a much better job with the idea, and so on, but at the end of the day, they didn't.
Verdict: loved it
My only real criticism is that it was all over a bit to quick.