I'm giving it three stars, although upon reflection it will likely climb to a four.
This movie is not for casual fans of The X-Files, or even newcomers. Those types will be bored, because the plot itself is not exactly compelling. As a standalone movie, it is not very good.
However, that is not why the diehard fans should stay away. This movie exists solely to explain, and call for further discussion, what Mulder and Scully have become since the last episode of Season 9. As a continuation of character development, this movie is a success.
As a diehard fan I am pleased, but if they decide to make a third movie, it had better involve alien conspiracy.