Now I will start off by explaining this: I watched this movie in Shinjuku one week after it came out. Being my first real anime I saw on the silver screen, plus the fact that I am a Steins;Gate fan may impair my judgement. You have been warned.
Steins;Gate:Fuka Ryouiki no Deja vu, or "The burden of Deja Vu" is the absolute ending to Steins;Gate. Need I say more? Yes? Well, let's get into the details while avoiding as many spoilers as I can.
This movie is a sequel, after the bonus episode "Oukoubakko no Poriomania", and it acts as a finale to the series, closing loop
holes in the story that were not closed, and giving it a last bit of closure. The emotional side of the story is almost masterfully handled, with the emphasis on Okabe and Kurisu's relationship to a pinpoint degree. I will say this though: The movie does not focus on the plot nearly as much as the character reactions, which is the strong side of Steins;Gate in the anyways. Unfortunately, that is probably the most I can say without spoiling anything in the story, as this movie is very tied to the Steins;Gate mythos.
The movie is rather short, less then 2 hours if I recall correctly, and it is a entertaining watch. However, like most anime movies based on a series, it would not hold a bucket of water if you watch it separately from the tv series, or have not read the visual novel.
Last word: Watch it if you have seen Steins;Gate. If you have not seen Steins;Gate, then go watch that so you can see this movie. You will not regret it.