It's a mature, intelligent tale of friends helping each other. Everyone's lovely and self-sacrificing. These are wonderful, funny, endlessly entertaining friends you'd give your eye teeth to have grown up with. Azu is loud, fun and always lifts the group's mood. Suwa is some kind of saint.
Despite this, though, it's also a slightly unsettling show to watch. I found it hard to watch multiple episodes consecutively, not because it's bad (it's not) but because I'd be fighting off premonitions of dread. The two main characters (Naho and Kakeru) are fragile. Naho's so self-denying that she describes her hobby as "housework" and she's almost incapable of doing anything for her own benefit. If she tries, her emotions shut her down. Her knee-jerk reaction to everything is self-negation. She'll tell herself that the screamingly correct action would just be a bother to someone else, even if in fact that other person is as bad as her and she's actually depriving them of emotional support.
"If I answer him, it might inconvenience him."
"No, I'm okay even if we don't go out."
A badly written Naho would have been unwatchable. She'd look like an idiot. Anyone else in her situation would have got on with things and made this a three-episode series. However she's not badly written. She's painfully relatable. We all have a Naho inside us. I've been one. We've made her mistakes.
She's so delicate and ready to break that you're always in slightly painful suspense. You want her to succeed. You'll probably identify with her, at least to some degree. However she's effectively a deconstruction of a character type who's so modest and self-effacing that they're incapable of acting when it's important.
At least she's better than Kakeru, though. In most ways they're as bad as each other, but the difference lies underneath. Naho's issues are due to her personality. Kakeru, on the other hand, is broken to the point of sickness. I think his mother did a number on him. Look at the way he cringes when confessing in ep.7, protecting his face as if he's expecting to be punched.
I haven't mentioned the thing that's probably first in everyone else's reviews! It's a time travel story, although the time travelling things aren't people. They're letters. The story's set in two time zones and two timelines. Most of the time everyone's 16, but every so often we'll see them aged 26 and looking back on their mistakes. (Mind you, the group includes a marriage and a baby, so it's not all bad.) The 16-year-olds send letters forward in the conventional way, by burying them in a box. The 26-year-olds send letters back with the help of some hysterically stupid technobabble, but you can ignore that as a necessary plot device. (A black hole in the Bermuda Triangle? Really? They should have just said it was magic fairies or something.) On the upside, though, the causality of all this gets justified satisfyingly, with parallel worlds being used to explain the mechanics of changing your past.
That's just letters, though. At the end of the day, Naho's letter is a push. It's telling her she has to change. She's got to stop hiding under her shell of invisibility and become positive. Regrets are an important thing in this story. Everyone has them, sometimes rightly and sometimes self-flagellatingly. Naho, though, actually has a letter from the future telling her what will happen as a result of seemingly inconsequential actions.
It's worth discussing Suwa, by the way. There's a triangle going on with him, Naho and Kakeru. Two of the triangle's sides are explicit, but it would be easy to infer a third from the characters' body language, decisions and stated feelings.
The visuals and soundtrack are unremarkable, alas. At best, it's a simple, summery art style. I found it quite pleasant to watch, but I wouldn't have guessed that Naho was supposedly less attractive than the allegedly beautiful Ueda-senpai. They all have manga-faces. You can't tell. What's more, there's enough bad art in eps.9-10 than I'd expect it to get redrawn for the Blu-rays. Looks like the show had a production crunch there. It won't have helped that the show's unusually long for a one-season anime, but that's not an excuse.
It's still a very good show, though. There's also a 2015 live-action movie that I'm now curious about. It's a warm show where everyone's lovely (except Ueda-senpai), but underneath that it's also a slightly uncomfortable show about how we fail to see ourselves properly and make bad decisions. It includes a sharp and surprisingly detailed examination of suicidal feelings. I think it's one of the few important anime of the year.