So I just finished this series not too long ago. Honestly I expected much worse but I wouldn't go as far as saying it was a good show.
The show is basically a vastly different alternate universe version of Fate/stay night where the Holy Grail War which both Fate/Zero and FSN focused on never happened, the servants used in the war are replaced by 'Class Cards' with special powers coresponding to the various classes (Think something like the Clow Cards in CCS) and of course Illyasviel becoming a magical girl.
It's pretty messy but the way it pays tribute to the other titles in the franchise is probably the best part. On top of that there's some surprisingly really good looking action scenes and some really funny comedy skits here and there.
Unfortunately there were a lot of downsides which lead to the show falling back into the realm of mediocrity for me. Lolicon fanservice is a given in the type of show it is so I can't really criticise it for that even though it was quite annoying. What I can criticise the show for is the fact that every time it tries to take itself seriously, it gets instantly worse. Forced melodrama, misunderstandings, brief forays into grimdark territory, you name it.
Once again I have to stress that it's probably not as awful as you may think a show like it would be. But at the same time, that doesn't make it good. Supposedly Season 2 is better but Season 1 on its own is painfully mediocre.