Is Fate/Stay Night worth a watch? Depends. The anime alters characterization and personality of it's important characters because it apparently shows characterization of the characters through ''visual story-telling'', which just happens to be the extremely ''bright'' idea of a director that brought us KnK 6 - his first project (UBW being his second) that also happened to be butchered to hell and was badly receieved by the fans.
So no, the anime isn't worth a watch. Both the old ones and the new one are terribly done simply because the old one was done by a horrible studio while the new one had possibility to be good, but was given to their worst director who happened to have an idea to make a first person story - the story and narrative that is carried out by the main character's point of view - into an ensamble cast story and ''lolvisualstorytellingthathasnochanceofbeinggoodwithaVN''.
Fate/Stay Night VN is very much worth
a read, yes, but the anime that isn't a standalone and expects you to have knowledge from the VN (they expect you to have knowledge from the Fate route) is not good unless you are a VN fan who is watching it only for the animation. In other words, it's fanservice for the VN readers and it's not really recommended to anime only people unless they don't give a shit about the story or characterization.
No, watch Fate/Zero first, after that watch Fate/Stay Night UBW, it makes much more sense that way.
In Fate/Zero the main character of fate/Stay night UBW is a kid, whereas he is much older in UBW.
No. Don't recommend this order to people. Because if they actually watch Zero first, they would spoil themselves on about 80% of the plot twists in Fate/Stay Night. Watching Zero before SN is like picking up a detective movie about a murder, skipping through the movie all the way up to where the detective explains how the murder happened after finding clues and things and
then skipping back to the start of the movie and watching the whole clue gathering, etc. and the whole main story that was supposed to be viewed before the murder flashback.
Either way, this time it doesn't matter since he already watched Zero first before you even recommended it.
Fate/Stay Night is still well written and extremely enjoyable even if you spoil a lot of plot twists with Zero, but it takes out the mysterious and crazy factor in it's ride that tones down the experience.