The problem with this film is that, plot wise and in terms of production values, it felt like just a regular monster-of-the-week episode from the series that was stretched to last longer. I found it to be a decent thriller, but at the time I was expecting something more. Then again, opening at the box office during the summer of 2008 at the same time as "The Dark Knight" was not a smart move in terms of distribution.
This is really nothing more than a television episode stretched into a film with barely enough suspense to hold your attention to the end. It could have been much much better given the interesting theme of Well's Island of Dr. Moreau meet Bulgakov's Heart of a Dog.
About on par with a below-average episode. While not as polished as the first film, there's some effective atmosphere with some nice chilly imagery from director Chris Carter and cinematographer Bill Roe, and there's some solid performances - but the plot is awkwardly paced and bogged down by ponderous subplots. Maybe worth watching for die-hard fans of the series, but ultimately underwhelming.
I'm in the minority of X-Files fans who actually like this. I think it's a great character study of Mulder & Scully that gets at the heart of why they do what they do and how their separate paths are unsustainable as a couple. Interesting stuff in here.