This will contain spoilers. I will try not to give too much away.
This movie is best seen not knowing anything about it. So take my word for it, and the merits of Joss Whedon and his co-writer Drew Goddard (he also directed the movie), and just go see it. It would help if you are a fan of or are familiar with the horror genre. But I don't think it's necessary in order to enjoy it. If you need more convincing or just want to get my thoughts after having seen it you can keep reading.
So here are some of the spoilers, or if you've seen it, some of the awesome things I enjoyed. Basically the whole thing with Bradley Whitford and Richard Jenkins. Nearly all of that was funny, not to mention adorably sexy when Amy Acker was on screen.
The most fun was probably when all of the creatures are let loose. I loved the shot where the second batch of swat guys went to the elevators there was all the blood and bodies with the three zombies eating someone. I was smiling ear to ear watching that. Then there was the merman and the unicorn, which were just fucking brilliant. I'll never look at a unicorn the same way again. I'll probably laugh my ass off every time I hear about one.
I want to see it again before I decide what I think the movie is trying to say. My first attempt at it would be that the whole horror genre fulfills some deep desire that humans have to see people get killed, which if I'm right is spelled out with the whole appeasing the ancients things. And it's human, not just American, as we can see by Richard Jenkins' character yelling at the Japanese in hilarious fashion. Whatever it means it was a a lot of fun. I'm glad we finally got to see it.