It's quite good, actually. It's been brutally cut back, especially at the beginning, but this actually works quite well. It's a much better movie than I'd expected.
Your favourite scenes will be missing, of course. It's a 107-minute movie, whereas the original computer game had about 10000000000000 hours of gameplay and the 2014-15 anime series had 26 episodes. Of course it's been chopped back. We knew that. Mind you, the anime series expanded greatly on the game and this is only an adaptation of that original, obviously. (I'm sure the production staff would have liked to have a time machine, but inventing and building one would have probably sent the movie over budget.) Unfortunately I can only compare this with the TV series, not having played the computer game, but I'll try to be fair.
Personally my favourite episodes from the TV series were the ones with Ilya and Caster's backstories. There's none of that here. Everything's firmly in the present. Drat. Nonetheless the film manages quite well with Ilya, all things considered, although this version of Caster is clearly much less interesting. Other stuff that feels simplified includes the Shirou-Rin relationship (only about twenty seconds of "friend or enemy?" tension), Saber's lack of motivations, Shinji (who's far less slimy), explanations in general, etc. The rules of the deathmatch aren't explained. We don't know why all these magic-using Masters and summoned spirit Servants are trying to kill each other. We never learn the "three strikes and you're out" rule of those magical commands. We're not even told properly what everyone's fighting for, even if the Holy Grail gets name-dropped in passing a few times.
You might want to have some familiarity with Fate/stay before watching this. The first five minutes in particular are risible. Ep.1 of the TV series is represented by a few disjointed moments glimpsed in the closing credits. The film lurches into Ilya's first appearance so clumsily that I almost laughed. It's incoherent. It's impressionistic plotting, trying to tell its story by throwing bits of it at a wall.
Once we're past that bumpy beginning, though, the film works. Take Ilya, for instance. We never see what makes her tick. We don't learn her history. However the character's still meaningful on a simple emotional level, just by virtue of what happens to her. The characterisation's more on the surface, inevitably, but even that level has enough power to convey emotional weight. Murder, betrayal, etc. This surface-level storytelling also helps Gilgamesh, who's a perfectly acceptable villain here despite being as one-dimensional as ever. He stands out less. There's less difference between him and the others. Plus of course he gets less screen time, so you're less likely to get bored of his shtick.
Similarly, this version of the Shirou-Rin relationship is thinner and simpler... but we can still see Rin's pain in that scene where she tears into Shirou and orders him to be more selfish. You can see what's in her head. The film also made me wonder if Archer might have been lying from the beginning when he'd said he couldn't remember his name. I think the scene's framing suggests that.
The fights can be spectacular, as you'd expect of a movie. Berserker is terrifying, for instance, although don't expect anything like his fight with Gilgamesh in the TV series.
It's less depressing. That's a huge one. They haven't changed the plot, but they don't dwell on the soul-crushing horrors nearly to the extent of the TV series. The film also spends longer afterwards being happy and uplifting.
It works. It gets through all the original's plot in the allotted time without feeling crunched or short-changed. The pace suits it. I think you probably need to know the mythos in order to understand what you're watching, while furthermore I suspect you'll see more emotion in these scenes if you already know the story and so know what's in the characters' minds. That said, though, unfortunately the game's fans see this movie as at best a decent but forgettable adaptation of a far more powerful original. I can understand that viewpoint, but it's not a level playing field. Thirty-odd hours of story vs. ninety-odd minutes. I think it's a pretty decent film and one of the better anime movies I've seen.