It's pretty good. I enjoyed it. It works. I didn't adore it, which makes it inferior to the anime, but it's still a good deal better than most live-action films adapted from a manga/anime series.
The plot's basically unchanged. Arata Kaizaki (Taishi Nakagawa) takes a pill that makes him look ten years younger and returns to high school for a year. It's good stuff. Go read/watch it. The main difference from the anime is that this film had more manga to adapt, so don't be surprised if plot beats happen earlier than you're expecting. If you've watched the anime but not read the manga (like me), then the film's second half will be mostly unfamiliar material.
They've trimmed down the story, of course, but the streamlining works. Some of the compressions could almost be argued to be improvements, on the principle of putting more focus on the best material by cutting out stuff you don't need. I wouldn't go that far personally, but it's certainly a much defter live-action manga adaptation than many I've seen. Even the one drastic change (the graveyard scene) still works emotionally in the new version. (Personally I think they've cut a little too much from the scenes around it, though, giving it less context.)
The worry, of course, was always the cast. How many of them sing in boy bands? How many were cast just for being pretty? One's doubly nervous here because it's a high school story, making it more likely that we're getting seventeen-year-old mannequins.
In fact, though, happily, we get out in one piece. There was room for improvement, but everyone's at least okay and does the job they've been assigned.
Crucially, Taishi Nakagawa is very good in the lead role. I'll be generous and assume that a couple of uncharacteristically telegraphed bits of business (checking his wallet in the restaurant at the start, rubbing his chin when watching Kariu and Ouga) are things he'd been ordered to do by the director.
Downright glorious, though, is Yudai Chiba as Yoake. He's especially fun in his early scenes, when he's being punchable. Seeing him in the role justifies this film's existence, frankly.
Those are the two actors I thought were actively good. Everyone else is okay, but there's no one I actually dislike. Admittedly Sae Okazaki's An is a null, but that's no fault of hers. The character's effectively been written out of the film, with nothing more to do than to be a walk-on semi-background character. She's nice and that's it. The actress isn't getting the opportunity for anything more. This on its own makes the film a poor shadow of the anime, in which I think An was my favourite character, but then again I'm pretty sure that whatever teen actress got cast wouldn't have been able to live up to the original. Maybe we dodged a bullet?
Elaiza Ikeda is fine as Rena Kariu. Mahiro Takasugi is similarly fine as Kazuomi Ouga and in fact I wished that the film had trusted his acting a bit more. His big scene with Kariu gets shoved in so abruptly that it made me go "eh?" and Takasugi deserved more of a chance to set it up beforehand.
The big question mark, though, is over Yuna Taira as Hishiro. Her problem is that she's playing a block of wood who doesn't emote. That's not an insult. That's the character as written. Furthermore, you can't accuse Taira of doing nothing. Watch her worst scene (the one where she's trying to ask for Kaizaki's mobile number). You can see what the actress is trying to do if you pay attention, even if I think she's misjudging how much she can and should be doing within the confines of an emotionless character. Oh, and her dance later is cute.
In short, the film's fine. It works. Doing young and old versions of the same characters isn't a problem at all, in case you were expecting a horrible production failure there. I have the odd nitpick (e.g. Kaizaki's huge apartment), but it's a decent, entertaining film with a heart.