Why some fans dislike the Alien mytharch I'll never understand. The typical explanation, that the alien mythology is confusing, and therefore boring, is simply incorrect. While the X-Files was on the air, I followed the storyline easily enough, but now that that series is on DVD, one can see just how well 9 years of the mytharc actually fit together.
Vienen is a nice return to the black oil storyline, which is really one of the most interesting and foundational plots of the X-Files.
If I had one complaint about the episode, it's the amount of time the writer and director decide to spend on Kersh. Is it really necessary to show him frowning again and again and again? Instead, the action of Mulder and Doggett dodging the infected crew would have been much more thrilling.
Having said that, it's a great episode, and reminds me of why I like Mulder and the X-Files: the willingness to push for the truth no matter its outlandish nature.