I'd never played the game but watched the first episode last night--maybe because I wasn't expecting much, I was impressed. The cast is compelling, all the characters are interesting so far except for Sakura, who's moe but just seems like your generic doormat otherwise (so far). Fuji-nee is awesome, she should be the main character ^_^. The premise of the series looks like it could be great, I can see why the game has a lot of fans (although they didn't explain much in ep1, I ended up looking up more info on the game after I watched it so I could figure out what the heck was going on). If you don't mind spoilers for the end of the game, this is a great website that goes into detail on the cast and each of the three game scenarios.
Some people complained the first episode was slow, but I didn't mind the slower pacing, I'd rather see more character development than fighting anyway. If I had to list things I didn't like, I wasn't all that impressed with the character designs--they just look flat to me, kind of like Wind ~ A Breath of Heart. Too bad they didn't hire Rokku. More explanation about the war and the masters/servants up front would have been nice for the viewers coming into this with no experience of the game (although since Shirou doesn't know what's going on yet, I guess the writers want to keep the audience in the dark too).
The "when you save one person you can't save another" line from Shirou's adopted father looks like it'll be a main theme of the series, especially if they go the Heaven's Feel route. I'm not sure how they can get all three scenarios from the game into one series though, granted I haven't played the game but it sounds like the scenarios are mutually exclusive? Based on spoilers I read it sounds like if you tried to put all three together sequentially they'd end up contradicting each other, since each scenario has different people responsible for events in the town... The game has endings ranging from happy to bittersweet to tragic so it'll be interesting to see which the anime's writers go with.
It's a pity Rin doesn't look quite as kawaii in the anime as she does in the artbook scan above, but that's probably too much to ask for...