Critters, like all great monster movies, can be summed up in a few short lines. Crites- small alien monsters- steal a ship and escape to Earth. Two shape-shifting alien bounty hunters are hot on their trail. Meanwhile, the Brown family children are going around engaging in small town mischief. It's probably only a coincidence that this movie came out two years after Gremlins, with its small humorous monsters and PG-13 violence.
First of all, the Critters are much scarier overall than the Gremlins are. There is some pretty graphic violence here, pushing the PG-13 rating as far as it can go. Second of all, this movie is extremely funny. I hate little kids in movies but the kids in this one are great. Finally, there are some amazing effects going on here. The introduction, which takes place on a spaceship, features a great set and some fantastic alien costumes. Then there is a really outstanding effect at the end of the movie that was probably not too difficult to pull off but still ends up looking great.
As a sort of mash up of Gremlins and your typical monster movie such as The Blob or what have you, it's a mixture that works very well and it would definitely be a mistake to skip it over thinking it's a cynical cash-in on Gizmo-mania.