Valerian is a reasonably enjoyable sci-fi adventure film. It is not, by any stretch of the imagination, actually a good movie, but if you're willing to ignore its more glaring defects, it's quite fun. By far its best quality is the visual spectacle. The advertisements quote a review comparing it to Avatar, which is an easy (and lazy) comparison to make - Valerian's primary alien design is a smaller, pearlescent version of Avatar's aliens. There is also a plot point or two that are reminiscent of Avatar, but otherwise Valerian has more in common with Guardians of the Galaxy or Jupiter Ascending.
Like Jupiter Ascending, it's really best not to look at the plot too hard. Luc Besson wrote the script himself, which might not have been the best of ideas. There is, for instance, a particularly egregious maid-and-butler scene (where characters talk about things they should already know as a way of telling the audience). The science is laughably bad, but if you were expecting hard sci-fi you should have been paying more attention.
Like the latest Pirates movie, the romance subplot between the two leads is somewhat problematic, forced, and unnecessary. It is, however, entirely typical, unfortunately.
This is a movie you'll want to see on the big screen to get the full effect of the visuals, but you might catch a discount showing, and make sure your disbelief (and possibly your critical thinking skills) are well and truly suspended.