(FULL SPOILERS AHEAD)
I have Fallout at the bottom of my M:I ranking list so was apprehensive about this, but wound up enjoying it quite a bit. It resolves many of the biggest issues with that film, notably that it doesn't take itself nearly as seriously and reverts to the Looney Tunes vibe of Ghost Protocol (at one point Ethan is threatened by a dangling grand piano); the film spaces out its action scenes and puts more emphasis on story, avoiding the feeling of being a numbing conveyor belt of disconnected stunts; said action scenes are fewer in number, allowing each to have more impact and more time to develop and be inventive within themselves, and with a much stronger sense of cause-and-effect progression compared to the equivalents in Fallout (particularly the interminable helicopter climax) feeling like one contrived scenario plonked after another for the sake of just having more stuff happen; the idiotic mythologising of Ethan is mostly gone, and the one big line that could be construed that way is played for laughs; Hayley Atwell is a very charismatic addition to the cast and brings a bit of normie energy which makes the film around her a little more relatable; Pom Klementieff is a fantastic henchwoman, with one big caveat (see below). I also enjoyed Ethan doing his dorky little magic trick from the original film.
On the downside, while it is paced much better than Fallout and entertaining at a minimum from start to finish, it is definitely longer than it needs to be and gets a bit tiring in the final act; the story is simple, which is fine, but the plot contortions within it can be needlessly overwrought (two versions of the Entity, a late change in its origin, several logistic questions) and the ultimate threat is too abstract (threatening the 'concept of truth'?) to add real stakes; while it's clear the emphasis is on the AI as the main villain, Gabriel as its human embodiment is rather half-baked, and like the return of Julia in Fallout, his connection to Ethan's past feels completely arbitrary; I like most of the main characters in this to some degree but there are too many of them, with the two agents chasing Ethan feeling particularly disposable, while Kitteridge and Denlinger should have been a single character; Ilsa, once again, is given absolutely nothing meaningful to do, and both her return and fate continue to feel like they solely exist because she was so fantastic in Rogue Nation; the only thing I genuinely hated about this film was that after being a fantastic henchwoman for 90% of the film, Pom Klementieff's Paris is turned good at the end against established character and for the flimsiest reason, seemingly just to deliver exposition that could easily and more convincingly have been handed out elsewhere. She also deserved a more bombastic death than she received.
Finally, something I'm conflicted about: I really like Hayley Atwell and her character (Grace) in this, but the role would have been so much more immediately evocative had it been written as Nyah Nordorff-Hall/Thandiwe Newton from M:I-2, arguably the series' biggest dangling thread as one of Ethan's former love interests. Like Grace, Nyah is a thief and she's outside the service, and her history with Ethan means their reconnecting would have a much greater emotional impact and make less ridiculous Gabriel's threat of killing one of two women Ethan cares about, Ilsa or Grace, given Ethan only met Grace a few days' prior (it would also make it far less obvious who is going to die for that same reason). The only change to the story would be that a white girl is needed to double Vanessa Kirby (who gives another enjoyably hammy performance) but were it Nyah who is killed on the bridge - which would have more weight as Ethan and Ilsa's 'romantic' connection was never remotely convincing to me - Ilsa could fill that role and be brought into the IMF, which seems a far more logical progression for her character (she really should have been the one to take the series over from Cruise when/if he retires).
Anyway, enough rambling. The original film and Ghost Protocol are still the top two in the series for me, but this and Rogue Nation are battling it out for the third spot, both being flawed but essentially enjoyable movies which do a lot more right than they do wrong. I also appreciated that this didn't end with some silly cliffhanger, but Ethan's mission from the start of the film being achieved as a stepping stone to the next one (though I'm far from convinced the story as seen so far needed two films to be told). Overall, good stuff.