There's no question Logan is a great superhero movie. That said, I can't sign on with the critics calling it the greatest of all time. In fact, it's not my favorite of the Wolverine movies.
The "yet" is important, because it's easily the best Wolverine movie to be released theatrically. In my personal opinion, it falls just shy of the extended version of its predecessor, The Wolverine, also directed by James Mangold. If Logan gets a similar director's cut, I may well reconsider, since most of my issues with the film stem from the fact it's too short. That may seem odd - Logan is already more than two hours long, and it features what may be the most deliberately slow pace we've ever seen in a big budget superhero production, but there's something missing here.
The movie's break-out star is Laura, a young mutant who gets adopted by Xavier and Logan. She gets plenty of screen time and some absolutely amazing moments, which is more than sufficient to make her a phenomenal addition to the film. But we don't really get enough to have a grasp on her personality or - more importantly - her relationship with Logan. Given that the second half of the movie is ostensibly built around that relationship, this omission undercuts the emotional resolution of the film. I left the theater without really buying her arc, which is unfortunate.
On the other hand, I also left hyped up on how bad-ass the fights were and blown away by the movie's cinematography, character acting, pacing, and tone. It's a well-made, unapologetically grim and violent near-future dystopian superhero western: how the hell does this even exist?
With a little tweaking, though, it could have been a masterpiece. Without that, it's just great. Only awesome. Nothing more.
Keep in mind the theatrical release of The Wolverine had a similar issue. Logan's relationship with Mariko was underdeveloped until the extended cut, at which point it became a much stronger picture. I'm keeping my fingers crossed there's a similar version of Logan in the works: I'd love to see it.
Even if that never materializes, this is already a fantastic - and bizarre - movie that flies in the face of conventional studio wisdom. It defies attempts to place it logically in continuity to a degree that has to be intentional. The original X-Men and Origins: Wolverine are referenced, but nothing else quite snaps together (not even The Wolverine - Logan lost his metal claws at the end of that one, but he's certainly got them here). You get the feeling that's how the director wanted it: this is a stand-alone movie that exploits seventeen years worth of character development but isn't beholden to time travel, soft reboots, or anything else. You won't catch me complaining: I love superhero worlds with or without continuity.
Ultimately, Mangold has delivered a superhero western to follow up his superhero samurai movie: the two compliment each other nicely. Logan is a great send-off for Jackman and Stewart that's absolutely worth catching in the theater (as long as you leave the kids at home - this is easily the most violent superhero movie since Watchmen).