They stick to the formula: there's a scary opening scene in which some supernatural force kills some poor schmo, Mulder and Scully run around talking on their cellphones, a new case allows them to stumble upon more clues about the big conspiracy, Scully does an autopsy and finds something really unusual (surprise), there's a chase, a hearing before their superiors, Scully is taken hostage and is put on the verge of death, Mulder has to go save her... yadda, yadda, yadda.
The movie satisfies because it gives you what you want and what you expect, only doing it on a bigger scale with better special effects. A huge explosion at the beginning, and a big James Bond-style finale inside a huge remote hide-out could only have been done in a movie. The picture looks great, the cinematography is a nice step-up from the murkiness of the show, and, more importantly, we get to hear Mulder curse.
The bad news is, because the film sticks so close to its roots, it also inherits the weaknesses of the show. Just as there is no final resolution each week, the movie leaves more loose ends than it ties-up. And while it looks and feels just like a big two-hour episode, in the final equation, it does not surpass any of the better series installments.
Non-x-philes probably won't be too lost thanks to some convienient, expository dialouge. Example: Mulder just about sums up the entire show in three sentences while explaining to a bar maid his exact reasons for getting drunk. But I doubt the film will create new fans. It's a sci-fi movie about humans versus aliens, and anyone who wants to see that sort of thing is probably a regular viewer of the show already. Unless they're really busy on Sunday nights.