I recommend playing Fate/stay night, the game from 2004, entirely (all three routes) before watching Fate/zero. You won't lose anything in terms of watching F/Z -in fact you'll appreciate many parts of the story more this way- but most importantly you'll be able to avoid spoilers by starting with the VN. Not that you couldn't enjoy the VN after watching Zero; it's just that you'd probably find it great, but relatively far from a masterpiece, and I feel this would be extremely regrettable.
My first impression of the anime was excellent. The pace is good, the music fits well to the plot and the visuals are stunning. As this is the prequel for one of my favorite stories of all time (FSN), my expectations were as high as they could be and on the surface, the anime doesn't disappoint. However, after spending considerable time reflecting on how well F/Z fares in terms of what I consider some of the most important aspects of a great story - the overall structure (consistency and arrangement), characters (and their development) and 'intelligence' (including depth) - it appears to leave much to be desired.
As an adaptation, Fate/zero is quite high-quality. The pace isn't too fast - unlike the adaptation for Fate (or the UBW-movie, Umineko etc), it wasn't created as a 'bare minimum' that rushes through the main points of the story, leaving out any finer detail. Thanks to the nearly optimal pacing, the atmosphere has been kept largely intact.
Unfortunately, the plot was born as a background story in the visual novel, and although Urobuchi did a fairly good job bringing to life the framework for the events that is set up in the game, the big picture ends up feeling shallow, even to the point of resembling that of a generic fanfic. All new characters lack depth, and with some important exceptions, many are stereotypical or otherwise uninteresting. In terms of development, only the protagonist receives satisfactory treatment. For some reason, Urobuchi even wastes some quite obvious opportunities for potentially interesting interactions. He might've found them difficult to integrate to the main plotline or perhaps distracting. However, I feel the story isn't complex enough to afford such simplification.
Even worse, the plot in F/Z tends to develop in such ways that are ultimately incompatible with constraints set by the portrayal of the events in the original. This leads to problems that are often handled gracefully, but sometimes they're solved forcibly enough to leave a mildly unpleasant aftertaste. I imagine that with some effort and creativity, perhaps involving plot additions, all of this could easily have been avoided.
I'm also unsatisfied by how important events are handled. There's often little to no build-up, and most of the more anticipated scenes end up anticlimactically. This necessarily makes for a more bland viewing experience, and probably could've been dealt with with relatively little work on the author's part. This may just be Urobuchi's 'style', but I personally don't appreciate it.
Some feel /zero features themes that are more mature than those of FSN, and while I understand the reasoning behind this, I see things the other way around. The newly added drama and conflicts (that aren't present in the original) are largely shallow and stereotypical. The deepest philosophical elements in the anime are the same as those introduced in the visual novel, but F/Z inspects them from a narrower perspective -- since the game consists of three noticeably separate stories, this is to be expected, but it is worth taking into account.
A less significant point I find disappointing is the relative lack of effort spent in making the battles entertaining. With one or two exceptions, not much thought seems to have been put into adding strategical flavor or deeper plot involvement.
As an overall rating, in an intuition-based analysis I ended up with the following:
Story: 74/100
Music: 70/100
Voice Acting: 90/100
Graphics: 85/100
Overall: 74/100
(I personally base the overall enjoyment mostly on the plot)
