Don't expect too much. (spoiler-free review) The main thing about this movie is that, whilest watching it, it only made sense to me because I watched the original series (twice even because I like it so much). A lot is left out, and yes, the main plot points are stuffed in there, but they're basically presented without context and a lot doesn't make sense at all if you haven't watched the original series. I can't quite give a good opinion on the story/art/character development because of this. It just barely contains any that makes sense without the context of the series. Besides that, it occurred tome that the movie looked like a literal edit of the series. I don't believe they've re-animated anything nor make it fancier (ex. how the Madoka movies were literally the series but with much better animation).

I don't want to completely bash this movie into the ground though, it just needs a little instruction I think?

Basically:
If you want to watch the movie because you watched the series, hell yeah! The great thing about the movie is that, the series was a while ago. If you want to go into the next movie I suppose you need the base the series/first movie shows otherwise you won't know anything about the world.

So, as a reminder of what exactly the series was about and who these characters were, I think this is a great thing, since you don't need to watch the whole series all over again.

HOWEVER,

If you want to watch the movie without watching the series first, I can strongly recommend against that. You'll miss out on tons of story and context and I'm sure a lot of things won't make sense to you since there's barely any worldbuilding in this.

I haven't watched the second movie yet, but I'm pretty sure the best way to watch and enjoy Kyoukai no Kanata is to watch the series, the first movie and then the second movie. This way, you won't miss out on anything, and if you are doing a marathon, you can even skip out on the first movie (though I'm not sure if the small fragment after the credits matters, because that IS new, contrary to the rest of the movie).

Either way, I'm not here to dictate people, and this is merely an advice ofcourse. If you enjoyed the series and you just need a thing to remind you what it all was about, this is a pretty great movie, since it takes away the need to watch the full series again.

If you're planning to watch this without having watched the series, I wish you good luck, because there'll be a lot of confusion ahead.

Have fun!