It's the first live-action film of Assassination Classroom! (The sequel is coming in 2016 and this one ends with "to be continued", albeit in a good way that doesn't stop this from being a satisfying film with a decent ending.)
Is it a good film? Yes, obviously. It's still Assassination Classroom. The word from people who haven't seen the anime or manga have been hugely positive. It's still lots of demented fun. It's a jolly, feelgood film about children trying to murder their teacher every day, because he's going to destroy the Earth. Koro-sensei's still the star of the show, being a planet-vapourising colour-changing octopus who can fly at Mach 20 and is also determined to be the best possible teacher to all his assassins. I enjoyed it. It's a fun introduction to the franchise. All the nitpicking I'm about to do doesn't change the fact that this is an entertaining adaptation of a laugh-out-loud story and you'd happily show it to friends.
However I still think you'd be better off with the 2015 TV series or the original manga.
To state the blindingly obvious, it's only 110 minutes long. The manga has well over a hundred chapters so far, each maybe twenty pages long. The 2015 TV series takes just over 500 minutes on the same material as the live-action film and you wouldn't want to lose a minute of it. The movie is faithful to the best of its abilities and it feels representative of the full story, but it's not the full story and yet it's not trying to be something else instead.
Admittedly the finale diverges, but not in a consequential way and we still end up in the same place. (It's quite sweet and likeable, though.) In other words, it's the same story, but less of it. So, yes, watch this film. It's a laugh. But it's not as good as the non-compressed versions.
It can feel bitty. Lots of abbreviated incidents, lots of characters, lots of murder attempts. Much of what's in the film is there simply because it's been inherited from the manga, but that's a long-form serialised medium. Characters appear, cause trouble for five minutes and then disappear again. Explanations have similarly been minimised, which was fine for me because I know the story but have made some newbies scratch their heads. They've wondered, for instance, why this ramshackle arrangement is the world's plan for killing Koro-sensei and why the children aren't worldwide celebrities. You might think a superbeing's promise to destroy the Earth would have more global impact than we see here. As it happens those aren't logic holes, but the film's going fast enough that I can see how a viewer might assume that they were.
Scenes and story beats are also liable to have less impact than they might have had, either for running time or for budgetary reasons. The live-action versions of those scenes will work, but they'll be less spectacular and/or interesting. Class 3-E's big set-piece elaborate assassination that approximately succeeds is far simpler and hence less ingenious in this film. Nagisa vs. Takaoka is a bit drab. It's also noticeable that they've killed any story arc that would have meant expensive location shooting, so there's no Kyoto school trip and no summer island holiday. They don't even go walking in the woods.
Mind you, there is an all-action opening with paratroopers jumping out of a plane for an all-out military assault on Koro-sensei. Guess whether that succeeds.
I missed the teaching side of things. The show's central gag is Koro-sensei taking the mickey out of attempts at killing him, but for me its heart comes from education. He makes his pupils better students and his colleagues better teachers. That's not absent here, but it's less prominent than in other versions. It's more about the colourful murder attempts. Kunugigaoka Junior High School's headmaster thus never meets the children or Koro-sensei and the film only pays minimal attention to the school's policy of institutionalised bullying. Other students don't sneer at Class 3-E. They don't get pushed around. The only interaction between a Class 3-E student and someone from another class is that between Nagisa and a movie-only character called Ayako Saito who has a crush on him.
There's also next to nothing about studying, exams, etc. In other words, Koro-sensei is a teacher without an academic setting. He doesn't get a chance to resist the school regime. Instead the script's looking for emotional depth by drawing attention to Koro-sensei's backstory and his promise to a girl who died. That's in the anime too, of course, but it's given more prominence here. In fairness, that works quite well.
Visually, they nail the most important thing. Koro-sensei looks perfect. He's a deliberately unrealistic CGI creation who's exactly like the cartoon character of the manga. He looks as he should. He moves as he should. I laughed at the bit where poison makes his head morph. However despite looking completely absurd, he also convincingly shares the screen with real people and objects. He feels tangible. You accept him as real. The CGI's seamless and they've done great work there. Koro-sensei is the film's gold-plated star, as is right and proper. Getting him right was the make-or-break of this film and it trumps everything else, really.
Apart from him, there's one terrible-looking bit with Karma and the cliff. That looked insanely bad. Being stuck in that run-down classroom area also could be argued to make the film look a bit drab. It's fine, though.
The acting is less bad than I'd been fearing. It doesn't significantly damage the film. The Japanese entertainment industry (film, TV and these days even anime) has a maddening love of casting pretty boys and girls who can't act. Unsurprisingly most of the main actors here are better known for singing in J-pop groups, while Ryosuke Yamada is miscast as the lead human character, Nagisa. He looks like what he is, i.e. a singer in a boy band. They've even given him the seeds of a romantic subplot with Ayako Saito. As it happens Yamada is at least quite short, but the film hides this fact from you. He's thus having to play against who he is, although in fairness at least he's making valient (if slightly eccentric) efforts to do so.
That said, though, the acting here is okay. Not great, but passable. Kanna Hashimoto (Ritsu) is bad, but you'll miss her if you blink. Kang Ji-young (Bitch-sensei) is variable. Masaki Suda (Karma) actually is predominantly an actor, although that said I didn't think he came across too strongly either.
It's the veteran Masanobu Takashima who gives both the best and the worst performances, actually. He's great for his first appearance, commanding the screen full-bloodedly and being enormously watchable. On returning later, though, he's become a ham.
Most important, though, is Kazunari Ninomiya (Koro-sensei). I still prefer the anime's Jun Fukuyama, but at least they're both on the same page and clearly better than 2013's Tomokazu Seki. I liked them both. Ninomiya's a bigger star, having previously played Private Saigo in Clint Eastwood's Letters from Iwo Jima. (Fukuyama in contrast is a full-time voice actor.) Ninomiya and Fukuyama are making very similar choices as Koro-sensei and you could almost mistake their performances. They're both good. Ninomiya could make me laugh with his "nurufufufu" snigger. However I think Fukuyama's more consistently playing it bigger, with Ninomiya being capable of fading into the background a bit with his line deliveries.
The film's a success, I think. It gets the tone, being happy, wacky and sincere. It completely succeeds with its on-screen realisation of Koro-sensei, it's highly entertaining and it's got a good ending despite going a bit panto with Takashima. Like the anime series, it also has a signature piece of music that makes any scene twice as funny. The anime's one used lots of piano, whereas this is a brass piece that sounds like a theme park ride. I loved both. The 2016 anime and film sequels should share them. This film is clearly the weakest of the three tellings of this story, I think, but it's still a laugh and it deserves to succeed.