It's much better than I'd expected. Live-action manga adaptations can be a hit-and-miss affair, but fortunately this one has no pacing issues because it's just adapting the very beginning of the story when Takeo and Yamato haven't yet become a couple. It's in no way a must-watch, but it's likeable and funny.
It's about as good as the anime, I think, although with subtly different strengths. They're both very faithful and immediately recognisable as each other, but...
1. THE STORY'S EMPHASIS IS DIFFERENT
Obviously, the story is a romance between Takeo and Yamato. In the anime and manga, they have a mild misunderstanding at first but soon sort that out. Everything thereafter is about this lovable, odd and inseparable couple. (Takeo is a hulking wall of muscle who could flatten trucks, while Yamato is a girly girl.)
The film, though, is all about that misunderstanding. Takeo and Yamato don't get together until the end. This changes the emphasis and makes the story more conventional. It's a standard romantic comedy. Will the boy get the girl? (If you think there's any doubt, then you can't have ever watched a film before.) This works. They've made a charming film. I enjoyed it and laughed a lot. It's all terribly nice, but at the end of the day we're just waiting for a foregone conclusion.
The manga and anime are more slice-of-life, being more about examining a relationship status quo than about the journey of getting there. I think that's more interesting, personally, because it's more distinctive. However I can see how movie-makers might prefer to go for the obvious narrative choice. It's more dramatic, for a certain definition of "drama".
2. THE ACTORS ARE HUMAN
Anime/manga Takeo is monstrously huge. He could almost put Yamato in his pocket. Obviously this was never going to be on the cards for a live-action adaptation, so Ryohei Suzuki is tall (six foot one) but nothing unusual. He's not a human gorilla. He's bigger than Mei Nagano (Yamato), thankfully, but we've lost the original's "huge guy, tiny girl" visual.
Suzuki was born in 1983, incidentally. He was in his thirties when he played a schoolboy in this film. I'm amazed, because he gets away with it. Mind you, it would have helped that Takeo is canonically a big ugly lunk with sideburns who's liable to be mistaken for his own father.
All this has an upside, though, which is that Suzuki is ridiculously good in the role. I was laughing throughout, not at actual jokes but simply at his Takeo-ness. He nails it. He's glorious to watch. Of course it's a fairly cartoonish role, with Takeo being a hot-headed lunk of a huge heart and very little brain, but Suzuki manages to keep it real even though he's dialling everything up to eleven. He's overacting (to exactly the right degree), while also layering in all of Takeo's feelings and making sure we're aware of what's going on underneath. He's doing things that don't fit together and making it look effortless.
I want to watch more Suzuki now. What else has he been in? Hmmm... Hentai Kamen. I have that on DVD and haven't watched it yet.
In contrast, Mei Nagano is merely good. She's the right age to play a schoolgirl, so we're talking about someone still in the early days of her career. Besides, Yamato's a far less extreme character than Takeo and so there's less excuse for flamboyance. She gets everything right, though. She's funny when she's with her schoolfriends. Her emotions are convincing. She does a solid job and I liked her. (She also talks at a human pitch, as opposed to Megumi Han's breathless/squeaky delivery in the anime.)
Oh, and there's also Kentarou Sakaguchi as Takeo's popular-but-distant best friend, Sunakawa. Previously, he might have secretly been in love with Takeo. There was something to discuss. Here, though, I don't think it's possible to deny the subtext expressed towards the end. It's subtle, but it's there. Watch his eyes. (Sakaguchi expresses it well, I think.) What is ambiguous, though, is its target. I'd agree that he probably is in love with Takeo, but the story would work just as well if he'd instead been suppressing feelings for Yamato... or even, at a pinch, for both of them.
This isn't a big-budget film. (If it had been, maybe they'd have made Suzuki bigger with CGI and trick photography?) That's not a problem at all and the film works great as it is, but the fairground fire (from anime ep.4) would have benefited from higher production values. There's a CGI shot with flames that aren't to scale.
It's a rental. I don't expect to buy a copy, but I'd recommend it in a flash to anyone looking for a escapist evening of happy romantic comedy. It suffers a tiny bit from the exasperation factor of watching idiots in movies who can't communicate, but... well, he's Takeo. He's a simple soul. Watching him and Yamato is lovely.