Short review: It's fun! Longer review, with (mildly) spoilery thoughts herewith:
So the movie is a lot of fun. I mean that positively, not as a euphemism for "you have to turn your brain off to enjoy it", but in the sense that the movie understands that, as an action flick, its primary job is not to be emotionally compelling or intellectually gripping, but to be broad-spectrum entertaining. So while there's some drama, mostly it keeps the action coming at a pretty steady pace, leavened with quite a bit of spectacle and a healthy dollop of humor.
I watched MI3 on DVD, and a good chunk of it was Tom Cruise emoting at maximum intensity about all the betrayal and reversals and whatnot that the twisty little plot had going on and, frankly, I was a little bored by it, because I'll be honest: I don't really care about Ethan Hunt and his feelings. The MI franchise is not about his character, it's about gadgets and the team and complicated plans with lots of interlocking moving parts.
MI4 seems to understand that a lot better. Cruise is dialed back to about 7 or 8 and spends a decent chunk of time projecting "taken aback" rather than "intensity". In fact, I think there may even be some stealth commentary on this issue in that at one point, Ethan Hunt is chasing after a bad guy and comes away from a tussle with a handful of mask -- the bad guy is someone in disguise! Uh-oh! Who is he really? And it turns out the answer is: another bad guy. The one they were actually trying to catch, in fact. I sort of felt like the movie was saying, subtextually, "Yeah, all that wait-whaaaat stuff from the last movie? We're not even going there. Bad guy. Chase scene. Let's have some fun."
The spectacle is really quite... spectacular. I paid a few bucks extra for pseudo-IMAX (mostly because of the timselot), and was not disappointed. I gathered from reviews that it's a movie that is well-served by seeing it in a way that it can mostly fill your field of vision, and I think it's true. There were some really lovely aerial shots, and the bits in the Burj Dubai deserve a serious vertigo warning.
It has the usual action-movie problems with physics (i.e., Ethan Hunt's bones are apparently made of diamond-titanium alloy) and excessively close calls, but nothing too egregious. And I can only think of one thing (the server room setup) where the illogic made me roll my eyes. Which is pretty good, overall, for Hollywood.
The new twist on the Mission Impossible setup in this movie, which I thought was quite clever and really liked, was things going wrong. It starts with the very first briefing tape failing to self-destruct properly, which is thematically cute, and gets worse from there. Nothing goes smoothly or according to plan, and in every situation, it's like the universe is just piling on to make things worse.
And it works! That's really a good summary of the whole thing: it works. It sets out to be straightforwardly entertaining, and succeeds.
Fun fact: you can get from sitting on the couch in our living room to sitting in a seat in the theater at the mondoplex in just 11 minutes if there's not much traffic and nobody in line at the box office. Win!