Of all of the D&D-related material not made directly by Wizards that I've found, this is definitely my favorite. The suggestions for classes are all quite true to genre conventions, and there are some excellent suggestions in here for adopting an Arthurian flavor into your campaign. The Knight core class is a lot of fun, as are the new prestige classes. My favorite is by far the Green Knight--it's definitely an NPC-oriented class if you're going strictly by canon, but it gave me a little bit of a rush to see the classic Gawain tale made manifest in a full stat block (They even get vorpal immunity!).
If I have one complaint, it's that this is one of the most poorly copyedited pieces of D&D material I have seen yet. Typos abound, even in section headings! I understand that this is a minor detail, but it shows a certain amount of carelessness in a book that is otherwise so full of great new material. I don't think this will be a make-or-break point for any buyer, but it did detract from my enjoyment of the book.
All in all, I've had a lot of fun implementing the Arthurian elements into my campaign world. This book will make a great start for all of you young squires ready to become knights, to woo fair maidens, and to battle evil the world over!