I started feeding my ferret Iams kitten food when I got her.  The ferret
book I had said that anything with a certain protein content would work,
and Iams had it.  When I talked to my first vet, she said that the kitten
was fine, but if my ferret started gaining weight, to switch to adult cat
food.  Eventually, she grew up and gained weight (within her first yr or
two) and I switched.  I have tried giving her ferret food, but she won't
touch the stuff.  In fact, she won't touch any cat/ferret food other than
Iams kitten, Iams cat, and Iams lamb and rice.
 
Now, one month shy of age 6, she seems fine (other than adrenal disease).
An examination proclaims her otherwise healthy (no blood work done yet
though), and her tartar is lower than is expected on a ferret her age.
Also, I moved and have a new vet who's very much a ferret vet.  I told her
I gave my ferret Iams cat food, and she did not tell me to do otherwise.
That's my experience with Iams.
[Posted in FML issue 3317]