I thought the movie was surprisingly good as well. The characters may not register all that strongly in the beginning, but once the craziness starts it definitely
grabs your attention. Especially with how freakin dark and gory it all is. And there are some pretty darkly funny moments towards the end as well.
The only real drawback is that the story beats are just so damn familiar by now. The movie does deviate a bit in some cool ways (like as far as which characters survive), but I still would have liked a bit more of that.