Moon Elves came out very early in the d20 boom, and to a degree it shows in its imperfect command of the system. However, it fulfills my requirements for what makes a good d20 supplement, so I'm going to give it high marks.
First of all, there are two versions in the zipfile, one marked "B&W", though both are color PDFs, with a color cover, color margin art in the introduction, and so on, the "B&W" version changes some color "journal entries" into normal text. This is a pretty primitive stab at printer/memory friendliness, but at least it's there.
This book expands on the race of elves presented in D&D3 (or Pathfinder's corebook, for that matter). Imagine that you were able to "expand" that section - it would pretty much be a book like Moon Elves. This is a good trait because it gives a simple puzzle-piece style supplement for a GM to include (or not include), and isn't dependent on other setting material (like the Forgotten Realms, for example.)
The language is very simple, aimed clearly at introducing players to roleplaying elements of an elven character. A considerable portion of the book discusses cultural elements of elves and provides good notes for players who wish to bring a cultural aspect to their roleplay.
There's the obligatory "new items" and "new spells" section, which can be used to add flavor to treasure or magic, but thankfully Moon Elves doesn't overreach as many d20 supplements do to providing new insanely great items. These feel like a natural counterpart to the society described. Similarly, the spells mirror the spells in D&D3 relatively closely.
The prestige classes are all right, but didn't really integrate with the cultural elements described in such detail in the beginning of the book.
Moon Elves is one of the more solid early entries into the d20 field - it's well worth a look even in these Pathfinder days. The price is exceptional for what you get. Who doesn't like elves? Nobody, nobody doesn't like elves. And that's a fact.
I'm reviewer tilting one star up for nostalgia purposes. D&D3 was when I really got excited about D&D again and Moon Elves is a supplement that will help you sustain and flesh out your campaign.
[4 of 5 Stars!]