Hi Jules,
 
>is it really good for them?
 
Well, I believe it's the best you could ever give them!  I've started
giving mice (and other whole preys) to my ferrets about 2 years ago and
they're all extremely healthy, they have beautiful shiny coats, more
muscles and less fat than most kibble eating ferrets I have met.
 
>and how often should they get one..  once a month?..
 
I would say as often as possible!  I can only afford one prey per ferret
per day because only one pet store im my area has them, they don't come
cheap and I'm on a rather thight student budget.  If I could, I would
solely feed them whole preys (not only mice but also rats, chicks,
quails, rabbit etc.).  Bob Church makes a lot of good points in his
diet series and explains all the benefits of whole preys a lot better
than I ever could.
 
You can look for them in the archives of the FML -
http://listserv.cuny.edu/archives/ferret-search.html
 
Hope you will take the plunge and give it a try!
Cheers,
Marie
Germaine, Hector, Lolita & Wallace
[Posted in FML issue 4595]