Yeah, I was expecting it to be a conversion kit, like Mk V Dreads or Tanks. Impressive. Most Impressive.
Evolution is an amazing thing, I'm thankful everyday for it.
You can take a single one as a LoW for Imperial Armies OR You can take 1 for every "Standard" Knight taken in an Imperial Knight Army, adhering to the IK Force Org, which I believe limits to 6 Knights, someone correct me if I'm wrong. So you could take 3 Cerastus' and 3 Knights from the Codex, points allowing.
I don't think it's the best model FW has ever produced, but it certainly isn't the worse. I think it's a nice compliment. It appears to be about the same size as a Warhound, but being combat orientated and "based" on the Knight Chassis.
I could see running 2 of them with 2-4 Knights. It'd be a pretty sweet, albeit expensive, force.