I picked up the Codex and a Knight kit and I am very happy with both. The Codex is slim and only has stats for two units, but is very well done. As you all know, I love the 40K fluff. And they did a good job with this codex. Great new artwork, nice mood setting fiction blurbs, and all the rules you need to use a Knight, including a quick recap of the relevant Apocalypse rules.
I would have liked to seem a few more units in the book, they mention the other types of Knights, and that each house has "men-at-arms" but no stats or list choices. It really is a ally codex, and to re-introduce the Knight Households. Is it too much to hope for the other Knights in the near future? Hard to say.
Do you need the Codex to use Knights? Not really. White Dwarf #4 is all you really need to take them as allies. To use them as a primary detachment you'll need the Warlord traits table in the Codex and the optional Knightly Ranks table. But I really like the book and recommend it.
The hardest part of new Knights is deciding on how I will paint mine. I am leaning towards the House Raven, Mechanicus ally, look but making a Freeblade or two does have an appeal. Either way, it will be a great ally for my Guard force. Until I get enough Knights to field a primary detachment that is.
The Emperor Protects
And looks good doing it