[MAY CONTAIN SPOILERS]
First of all, let me say that I think season 8 is too often automatically dismissed by fans simply because of Mulder not being around so much, and believe me, no one's a bigger Mulder fan than myself, but to be honest, season 7 was stale, and at this point in the show, I would agree with Duchovny's attitude at the time, that there really wasn't much left for the characters to do. All of their major story arcs had been more or less resolved, and while 7 had its highlights, the "magic" was, in a tangible sense, gone.
That being said, what better way to totally shake up your show than to have one of the two main characters gone for half of the season?! Even those who ended up hating the result were probably there every week wanting to find out what was going to happen - I know that's the sense I had, and that's one of the best things season 8 has going for it; it broke the mold and took some risks, and I think this alone made it MUCH more interesting than the bulk of the season before it.
It starts off, in my opinion, the best way it possibly could - those personal scenes with Scully coming to grips with what has happened while that haunting season 8 music says more about her situation than any dialogue could have. The introduction of John Doggett and the gradual progression of their relationship was expertly done, I thought, and he was the best thing that could have happened to the show, under the circumstances.
Robert Patrick's top-notch acting aside, Doggett's character - while a challenge to Scully in their paranormal investigations - almost immediately became one of the pillars propping Scully up during this dark time, a loyal protector when she needed it most in a way that Mulder never really was (his curious wrecklessness put her in harm's way more often than not). It was a very cordial relationship, to be sure (instead of the "Mulder" and "Scully" personal casualness she shared with her old partner, John Doggett was always "Agent Doggett" to Scully, and she was "Agent Scully" to him - there was a clear respect for each other on a deep level (even despite serious differences regarding the work), but they kept their relationship professional and platonic at all times).
Gillian Anderson also does a great job, as usual, evolving Scully's character and taking the viewer along on this bizarre journey.
Even so, the stories themselves are somewhat hit and miss. I tend to think they "hit" more than they missed in this season (highlights include "Within," "Without," "Patience," "The Gift," "Three Words," and "Essence"/"Existence," which probably should have been the series finale - oh, and the worst of the bunch would have to be "Surekill" and "Salvage"; the rest are somewhere in the middle). But what's strange is that, to me, the writers were doing a much better job in the earlier part of the season, before Mulder returns. As soon as he's back - when we should be cheering contentedly - I find that I cannot help but feel this is not the Mulder we know and love, that even though there are some good bits here and there (the season finale, while I have gripes about parts of the plot, is pretty much the last feeling of "classic X-Files mytharc episode fun" you're going to get for the series, what with all the late nights and parking garages) the soul of the show really is gone, at this point (this is the same problem I have with Scully in season 9 - she's just not herself. She becomes bland and lifeless much of the time, but then, that's pretty much the vibe of the whole ninth season).
There's a lot more to talk about (Kersh is back! The only season without Smoking Man, what the heck is the deal with the baby? Why is Reyes so annoying?), but I'll leave the rest up to the viewer.
Overall, I recommend this as, if nothing else, a fascinating case study on how a show can try to adapt when the basic formula for its success is drastically altered. You'll see some laudable achievements as well as some staggering examples of lackluster lethargy, but overall, I think this season deserves a lot more credit than it receives. For the most part, it managed to retain that elusive "X-Files" quality that made the show so great, and it as a whole, it's a lot more fun than a show in this tricky situation should be!