I played Kans for years but wasn't playing much of 6th to see for myself just how bad things got. I did read about it though. Kans were amazing. you had to learn how to use them though. They may be dead ard' again but we'll see how it goes. The key has been, and is likely to remain, protecting your kans. The KFF and cover were key to that. Again I have no practical experience in 7th regarding Kans.
I can say that My Kans were used to cheaply shield my foot slogers across the board while providing more fire power on to enemy targets. I know that Kans have a load of weapon options but the only two I ever use, so far, are Big shootas (one for every kan) or grot zookas. Grot zookas are a little better being blast weapons that have several shots so 3 becomes 6, very good even when you miss a few, those blasts have to go somewhere after all.
A lot of it will boil down to how you play you army, what you put into it and you play style.
I was running 9 kans and some dreds. Supported by loads of boys.
With 7th though and given a large amount of points I can see running quite a few and being very effective. But that would depend on 2 things. 1 running multiple detachments, I plan to run no less then 2, and of course the new codex. Kan Mob size may go up. in which case they might be better as one maxed out slot and then what ever else for the rest of your heavies. Yet another twist would be multiple detachments where each kan mob was very small. like MSU or min maxing just to make it much harder to kill them off. I haven't worked that one out as a list just yet though.
Hope this helps a little.