Ferrets have an odor all their own, just like any pet.  Some people
like the musky scent, a few can't stand it, and most are in between.
(Personally, I think it's much better than wet doggy smell or cat box
stench.)  If the ferret isn't yet altered [9.1], having that done will
cut down on the odor a lot; whole (un-neutered) males, particularly,
have a very strong smell.  Young kits also have a peculiar, sharp
scent which they lose as they get a bit older.

Descenting a ferret [9.1] doesn't change the day-to-day smell.  Only
the scent glands near the tail are removed, which prevents the ferret
from releasing bad-smelling musk if it's frightened, but doesn't stop
the normal musky oils which come from glands throughout the skin.

The two big things you can do to cut down on your ferret's odor are to
bathe him less -- yes, less -- often and to clean his bedding more
often.  Most of the musk stays in the cloth, on the litter or paper,
and on your floors and furniture, not on the ferret himself.  Cleaning
them can be a big help.  Also, right after a bath the ferret's skin
glands go into overdrive to replenish the oils you just washed away,
so for a few days the ferret will actually smell worse.  Foods
containing fish may make your ferret, or his litter pan, smell worse
than those with chicken, lamb, etc..  You may also find that your
ferret smells more during shedding season in the spring and fall.

Some people have had good luck with Ferret Sheen powder and various
air filter systems.