I liked the Doomsday Device, but I agree its use is going to be hit or miss. It seems more like an Ork device than a Chaos one though.
Karnival of Pain is a little meh. The benefit is nice, and the extra points are nice and low too; DE have so little as far as formations are concerned that it'll probably end up being used, but it still seems like a minimum effort, minimum thought, half assed production.
The Necron Shroudweaver war cell is a little better, but not much. Requires a huge point investment (which is fine if you are already planning on playing those units) but a 12" leadership test, night fighting rules and a marginal strategic asset for an additional 200 points ? ? ? On the face of it, it seems very high, especially once you compare to what the same number of points will get for other formations. I'll have to play test it to see if it's as bad as I suspect it will be.
On the other hand, the Eldar Shadowwalker War Host is excellent. Small point, good but not overwhelming advantage. I like it a lot.
My favorite is probably the Tau Long Range Infiltration Unit though. I like the idea, the points are about right for the benefit and the whole formation feels right for the Tau.