To start, the kits look awesome, and seem to have a ton of bits to choose from; four head variants, a couple weapon options, and assorted paraphernalia. As had been rumored, the price point is $140, and they're up for preorder on GW's website.
As to the rules, they are pretty straightforward, and not much that hadn't already been leaked. You can include them as a Primary detachment (though no force org - so no scoring units) featuring 3-6 of these bad boys, or an allied contingent of up to 3. Interestingly, there's no Allies chart I've seen referenced. It would be implied to maybe match Inquisitorial allies chart, but not clear cut by any means.
There are two kinds of knights, with no options.
The first is the Paladin, who comes in at 375, with a Rapid Fire Battle Cannon (2 shot, normal profile) and two Heavy Stubbers, along with a Reaper Chainsword. He'll be firing off a couple pie plates a turn, but where he's going to really do work is in Combat, using that D weapon, Stomp Attacks, and a S10 HoW.
The second version is a Knight Errant, who is 5 points cheaper and loses a Stubber while exchanging the battle cannon for a Thermal Cannon, which is a 36" big blast Melta weapon. He should put the fear of the Emperor in opposing vehicles, and, like the Paladin, will do the most work close up in combat.
In addition to their offensive output, they boast decent defensive capabilities, with the Super Heavy rules keeping them from being one-shot, and 6 hull points, they're tough, but not insanely so.
All in all, I think the rules are extremely balanced, and they might be slightly overcosted. That said, fielding 3 of these could prove a problem for many people to handle. The slippery slope comes with, since they use Escalation Rules, could you field these and still have Escalation banned? I'm curious how this will shake out, particularly in the Tournament environment.