They are the best bar none, the thicker the better. No problems with expansion since it is only nickel added to steel. If you can afford it go for it, you won't be sorry. Stainless handles the heat better without the decarburization problems that plagues mild steel in the same application. I think all the manufacturers should switch to all stainless construction just for durability reasons alone, if everyone bought them they would be cheaper.
The also seem to take longer to cool off kinda like a cast manifold due to the conductivity difference.
The Jet Hot coating seems to work well also. Not sure of the insulating qualities and if it helps prevent decarburization of plain steel.
Willys might know? He has had good luck with Jet Hot on his Willys headers.