I've only watched 5 episodes but this is what I've gathered thus far.
If you've played the Fate Extra games, then... don't expect that. The characters and the designs are from the first Extra but the setting is an AU. Although it also seems as though the Moon Cell Holy Grail War may have occurred before the setting of this anime. So get ready to be introduced to a different virtual reality, some kind of restart? I'm not 100% sure yet.
Also, I would like to add that Saber (Nero) uses Imperial Privilege which is a skill that she has in FGO (and I do not believe she has that skill in the game).
What I love about this series so far are the characters. Nero especially. Nero has this charisma about her which shines in the animation. I actually played Fate Extella recently and I had mixed feelings about her in that game, but I'm loving her in this. Shinji isn't too bad in this either. And Twice looks so good.
It's hard to form any real opinions with 5 episodes, but I'm enjoying it thus far and will recommend this to other Fate fans. I think it'll be good for non-Fate fans as well, as the setting is different than the main plotline.