In a nutshell: (ha, get it? Nutshell?) Anyway, if you loved the show, you are going to love the movie. It's got everything you want from one of the classic episodes: a great mystery, paranormal phenomena, tension between Mulder/Scully and all of the rest of the FBI, and, of course, the beautiful and haunting portrayals of Mulder and Scully by Duchovny and Anderson. I loved it and felt VERY satisfied when it was over.
If you did not watch the show, I think you would still enjoy the movie, possibly even more than the first movie. This movie doesn't require any knowledge of the show's background. It's a smart horror flick and it'll make you drop your popcorn more than once.