As a precursor, I would warn people that this review is from an irredeemable player of the visual novel. FSN is based on a visual novel that I adore and as such my review comes from a fan that has the original source to compare it to.
Fate/stay night is the story of the Holy Grail War, a secret competition held between magicians to obtain a device that will grant them any wish. Magicians summon Servants, spirits of heroic beings from myth and history, to do battle against other magicians in the hopes of attaining the Grail. Shirou Emiya, an amateur magus with no knowledge of the Grail is fatefully swept up into the war when he accidentally summons the Servant Saber, strongest of the seven Servant classes. Shirou becomes determined to win the competition to protect the unknowing public from the dangers the war presents in a bid to make true his dream to become an ally of justice.
The movie Unlimited Blade Works is based on the second of three routes that can be taken in the game. While the anime series predominantly focused on the Fate scenario with little references to UBW and Heaven's Feel, this film is all UBW. Unfortunately, it tries to cram in the same amount of material the anime covers in 24 episodes within a movie that clocks in under two hours. If you are not familiar with the source material or even the anime, you are going to be entirely lost with what is going on with the plot.
Technically, this show has a few things going for it. The budget of the show compared to the anime series shows, as does the couple years difference in animation techniques since the series aired. Unfortunately, it has all the same problems DEEN series do: some off-model work sneaks in, and some of the graphics can be distracting. As a sword fanboy, I also got rather distracted when the swords Gilgamesh spams in one scene clearly includes Elven and Rohirric swords from The Lord of the Rings films. Music is a different matter, as the epic quality of the series only improves with the soundtrack here.
As a fan, however, I have to shy away from positively promoting this. While I enjoyed it on the whole, part of the enjoyment was merely from knowing what would happen and wanting to see it in glorious high-def animation, rather than as a viewer gaining an anime experience. Because of the length, much of what makes the plot intriguing and characters interesting is sacrificed. The spectacle is great, but dramatically speaking, it falls far, far short of its source material. Additionally, the official localization in English is a tragedy, with many of the lines losing their significance due to how they were translated. One such instance in which Archer berates Shirou was subtitled as "You're still green in the bones!" rather than "Your ability to conceptualize the framework is still weak!" The latter is actually a partial hint, rather than merely an insult. Some of the subtle references the movie still maintains is further lost by the translation.
Overall, while my own personal score for the series is high, as a review I cannot say this comes as the highest recommendation. Really, your best bet is to check out the anime series first, or better yet check out the visual novel before anything else.