First off, as YT said, the animation was absolutely gorgeous. The Foley effects were also amazing - the ambient sounds of the empty world around them were incredibly atmospheric and spooky.

As for story, it wasn't necessarily anything ground-breaking, but it was very good. There was one "false ending" that I thought was a little off, but elsewise the pacing and action were superb. It was pretty short - 79 minutes, I think? - but it didn't feel short at all, to me.

And yes, it's pretty grim, definitely PG-13 (but not really a hard PG-13, if that makes sense). It's not a nice world these little guys find themselves in, and I jumped and cringed a few times myself.

Recommended.