It's a live-action TV version of Anohana... and, alarmingly, a two-hour special, not a series. Does it do the story justice, or has it been compressed to death? Answer: the latter. It's reasonably good TV and I'm sure the audience enjoyed it, but I see little point in watching this when the original anime still exists (and is for good reasons much better-known).
Surprisingly for a Japanese live-action TV drama, though, the cast isn't the problem.
It's the script. There's probably no good way of turning the original into a TV film, but squashed is still squashed. (The 2013 anime sequel/compilation film does a much better job than this, for instance, albeit with a flashback narrative structure that allows more freedom.)
Firstly, they've compressed the elapsed time span inelegantly. Jintan and Menma spend so little time together that their eventual love confession feels as if it's about seven years ago, not now. This breaks Menma's line about "the kind of love where you want me to be your wife". Similarly, Jintan still does all that part-time work to buy fireworks... but this barely lasts a minute or two on-screen, with no attempt made (e.g. a montage, a change of season) to suggest a longer time period.
Secondly, the female characters have suffered. The original felt like the voice of a female writer, whereas this feels like (and is) the work of a man. (All the anime characters were excellent, admittedly, but Mari Okada gave more layers and quiet depths to Anaru and Tsuruko than she did to Jintan, Yukiatsu and Poppo. She did less with Menma, but caught up there in the 2013 film.) Here, though, Anaru and Tsuruko are leftovers. You're almost wondering why they haven't been dropped altogether. "There's no replacement for Jintan" makes Anaru sound like a fantasy figure, not a person. As for Menma's mother, they've made her insipid.
Note the way they've rewritten some of Menma's letters at the end to be less subtle and more on-the-nose. That was necessary for Anaru, for instance, because her characterisation's so thin here that the real letter would have been reacting to something that's not there.
Oh, and they mess up the flashback of child-Poppo seeing SPOILER. The voiceover says he witnessed it, but the child actor clearly arrives too late to do so.
That said, the cast are a pleasant surprise. They're not perfect, mind you, especially in the early, establishing scenes. Childlike ghost-Menma didn't work for me. The acting in the lighter, funnier scenes is clearly worse than what Tatsuyuki Nagai gave us in the anime. However everyone really goes for the heavy emotional scenes, which is all-important for a story like this. They carry it. Thanks to the cast, this film is a lot better than it probably deserved to be. I believed in Jintan not wanting Menma to go, for instance. That felt true.
Even the small children are pretty good, all things considered. For example, I laughed at the flashback moment of child-Menma indignantly demanding a promise from Jintan's mother. My favourite actor here, though, was Fumiyo Kohinata in the small role of Jintan's dad.
Jun Shison is distractingly pretty as Yukiatsu (and his hair annoyed me), but he does a good job. Minami Hamabe is okay as Menma. (Personally I think the character looks better when white from head to toe, including her hair colour, but that would have looked unconvincing in live-action. She's no longer half-Russian.) Yuta Takahata is a satisfying Poppo. Yo Yoshida is a disappointment as Menma's mother, but she's not being given the material.
Incidentally, I nearly transliterated Anaru's name as Anal in this review, as it probably should be. Unfortunately, it reads badly. "Anal's good." "We needed more Anal."
There are things I like here. The song has emotional force when used as incidental music. The young cast's a lot better than I'd expected. (Hardly any of them sing in a band, for instance, although some of them are also fashion models.) The film's also making efforts to be faithful, e.g. the director often using the same shots and framing as the anime. It's a pretty good tearjerker. However it's also a bad version of Anohana.