An uneven episode that is nontheless entertaining, especially if you like the Lone Gunmen characters. I understand why they decided to go with Byers as the lead Gunman but he is so wooden in his performance (most likely intentionally so) that the "watchability" of the episode suffers a bit. The peripheral and sporadic involvement of Mulder also distracts from the energy of the episode.
There are a lot of plot holes. How could Susanne Medeski have escaped detection long enough to consult with the Gunmen and tell them her story when she still had the bug in her tooth? Why does X allow the Gunmen to live, especially after body-bagging one of his own agents alive? How could Mulder have survived such a concentrated dosage of the gas?
I enjoyed this episode quite a bit on my first viewing but I suspect that it may not hold up well on a second viewing. A little of the Gunmen goes a long way, they are among my favorite characters and they are an essential ingredient to the series, but I think they are more effective when used in a supporting role.