The comparison to The Fifth Element is very easy to make as the visual style of Valerian is very similar. So much so that it could easily be another story set in a different part of the same universe.
The story its self isn't nearly as visionary, Valerian (Dane DeHaan) and Laureline (Cara Delevingne), are two special space operatives, working for the government, set out to retrieve an item of extreme importance that could determine the fate of the City of a Thousand Planets. Things go don't quite go to plan, chaos ensues.
Though the plot is pretty standard, get the item give it the right person/people/alien before everything goes to crap, it's really the journey, the gorgeous visuals, and ideas that will keep you engaged for the majority of the film.
I've read other reviews where the general consensus is that this movie has a great first half then falls away towards the end. I tend to agree with those reviews, though it isn't a failure by any means and the end isn't unsatisfying at all. Generally it's a solid film. I feel the first half seems stronger simply because you're presented with so many new and cool ideas, whilst the second half is focussed on bringing the story together into its final act.
I do feel Cara Delevingne is short changed some what by the movie's title which puts the focus on Dane's character, Valerian. The two of them pretty much get equal screen time and are both very capable characters, saving each other and both doing kick-ass things all through out the movie. I guess 'Valerian and Laureline' (as per the comic title) would've made this sound like too much of a romance picture - even though the two are more than just friends right from the start.
Speaking of Dane, I'm not a big fan of his work. He was absolutely fantastic for Chronicle (2012) but doesn't quite work for me here. Possibly because he looks too young and sounds too 'emo' to be the capable, special operative he's supposed to be. He never feels threatening to me. He also doesn't have quite the comic timing and charm needed to sell some of the dialogue as natural (in the way, Bruce Willis does in The Fifth Element).
That aside, he's fine in the role of Valerian but maybe the movie should've been called 'Laureline and the City of a Thousand Planets' because Cara does a better job of selling the idea she's a capable special operative.
Beyond that I can't really highlight too many other issues I had without spoiling the story - other than to say one character's fate seems a convenience to save on the special effects budget rather than a necessity of the story - it's not terrible but it's obvious keeping the character around would be problematic.
As I said earlier, this movie is all about the journey, the visuals, and the ideas. Watching how the City of a Thousand Planets evolved at the start of the film is pretty engaging in itself (all to David Bowie's song, Space Oddity), even if it isn't actually essential to the story other than to understand why the city is so varied in every way.
If you are a fan of The Fifth Element, Valerian is going to be right up your alley. The story and its twists and turns are not nearly as well crafted as that film but the visuals and world of the movie certainly are (in some cases they're even superior - as they should be for a film made 20 years later).
Overall I did enjoy the film. I thought the two hour and seventeen minute runtime was going to be an issue but by the end it really didn't seem that long.
Well worth seeing on a big screen.