For the most part, I was disappointed with the original contributions and characters of Fate/Zero. Most of what I enjoyed were ideas from Fate/Stay with flashier animation (i.e. Saber's character, the Excalibur sequence). I completely agree with the maggot thing/character, honestly I forgot that character was even in there. Rin was gutted, Gilgamesh was amplified into a massively over-arrogant character who talks for hours on end, Waver and Rider were mostly pretty boring to me.
That said, there were definitely a few wow moments scattered throughout the series such as the first Lancer/Saber duel, and Natalia's story was pretty interesting I thought.
Fate/Stay also had some dry spells and sort of lame high school anime-esque frills. It's probably not my favorite anime of all time either, but it's one of the rare anime where I basically enjoy the full thrust of the story from beginning to end, and a lot of the characters both enemy and friend had tremendous vitality and tragedy in their stories such as both Caster and Assassin. Even Rider too, honestly. Kiritsugu I thought was interesting in both series.
Also that date episode with Saber. Plus her whole "death" at the end, so sad.
I think it's underrated personally, probably because there isn't anything attention grabby like a 100000 year old android/zombie with a chainsaw doing aerial battle with gothic transvestite wielding a honda, although as I've said there are some dry spells and things like that.
At any rate, it doesn't really strive for them super-complicated-philosophical-epic heights like Fate/Zero, but it's a lot more stable and sound artistically in my opinion. I don't think Gen Urobuchi's nihilism meshes well with the fate universe, which is already sort of grim and desperate by itself, in contrast to PMMM where it's slicing the most brazenly optimistic magical girl genre into pieces.
In my opinion, anyway.