Overall Rating 6.2 out of 10 - Flawed show with few good points here and there.
You will hardly lose anything if you start from episode 18.
"A worthy effort, but futile."
This show has occasional bright moments and occasional good characters. Sakura, Medusa, Medea, Shinji Matou and Gilgamesh are the most deserving of the cast. It has occasional great moments in story as well as attempts at philosophy and existentialism. However, all these efforts drown in a sea of mediocracy that is Shiro and Rin.
Ostensively this show is about Saber and sometimes they do remember about this fact. In fact, closer to the end they even attempt at building the romance between her and Shiro. However, it eventually gets to nowhere. I will come back to this later.
However endless Rin's lecturing, that take around 50% of the show, turns the show into annoying slog. Every time she is on screen makes the show Evangelion level trash. Theoretically she is knowledgeable mage who can think of solution to any problem. However, all her solutions are like ham-fisted piano in the grass. The whole show is like never-ending Rin ad or Rin development problem (a reference to Katsura development week in Gintama). All these efforts are futile. Nothing better summarizes Rin then what Kirei told her in Fate/Zero.
Saber is the most confusing character of the show and the only one I could make a concrete opinion of even by the end of the show. At times she is likable with her self-sacrificing and devoted ways. The other times she is obstinately stubborn. She wanted to stay with Shirou yet refused that out of sense of duty.
Shiro himself had all he needed to take Saber for himself, but just denied himself the pleasure. Shiro is one of the most illogical characters of anime, together with Shinji Ikari, even Shue from Guilty Crown has an inch more logic in that empty head of his. This show wants to draw parallels with Kiritsugi's decision to destroy the Grail in Fate/Zero. However, Kiritsugi only made Saber destroy the Grail after he has seen the kind of miracle, Grail was willing to offer him and rejected it because it's too far from what he wished for. For Shiro it's just a pure self-denial.
In the last few episodes Kirei talked about how hardships and privation make people better in his opinion. Again, a false parallel with Fate/Zero. In Fate/Zero Kirei merely wished to see people suffer, he knew full well it will not make them better, it would only make them worse. He oversaw Karia Matou killing Aoi Tohsaka out of jealously after he realized that she does not appreciate what he is doing for Sakura. Yet here in Fate/Stay, extensively more mature and experience Kirei sprouts this nonsense about people getting better through suffering. How did he manage to degenerate so much over the last 10 years. That is a rhetorical question. Fate/Zero had a better writing stuff, directed by Gen Urobuchi, so of course it turned out better.
If you want to know what hardships and privation produce you should watch Jigoku Shougo as well as Pain's and Garra's Arks in Naruto. Suffering produces Jigoku Tsushin clients as well as people like Pain and Garra who will make world know pain through massive exterminations. All that suffering in my life did so to me, so I know what I am talking about. If you want a real-life example, then take Adolf Hitler. All the suffering and privation in his life made him plan Holocaust and other Nazi crimes. Once in power he put these ideas into reality. He thought Jews were responsible for bad things tha happened to him and he made them pay for that.
"World shall know pain."
I guess as epilogue I will write some about characters I did not mention above.
Shinji Matou is the character that makes most sense in the cast. He reminds me of myself.
Sakura is nice domestic girl who is soft and gentle. She can clean and cook. And MVP girlfriend material.
Medusa/Rider is the kind of self-sacrificing character I can like. Unlike Saber she is loyal to her master and sacrifices for him rather than some abstract notions of duty.
Medea/Caster is very polite and accommodating person. She gives me TrueHellDei vibes. She is paired with subservient man and has too little screen time to show her submissive qualities enough. However I think she can make for a good submissive woman, if Gilgamesh was her master for example.
Finally, Gilgamesh is somewhat ruined here. He has no opportunity to showcase his brilliance and wisdom like in Fate/Zero. So, he only showcases his arrogance and pride. Other than his final words to Saber he does not say anything stupid though.