I must admit to slight disappointment. It's a pretty good movie and I liked watching it, so I'm not on the outside looking in (like with Monsters), but I wanted a bit more from this. It might just be a matter of expectations. Specifically, I don't really think this is a horror film. It definitely feels like an 80s kids adventure, but it mostly lacks even the danger of Monster Squad or Explorers, let alone something like Gremlins.
But it is really unique and a lot of fun, and if the premise appeals, you should see it. Just don't expect horror.