Pros:
1. Very playful, smart, and affectionate if trained well and handled regularly from a kit.
2. Enjoys human interaction, and is often good with other animals. Mine would romp with my cats and small dogs without even the tiniest hint of hostility between them.
3. Can be trained to use a litter box.
4. Very fun to watch them run around the room dooking, dancing, getting into mischief and otherwise enjoying life.
5. Can be a great apartment pet for someone whose landlord doesn't allow dogs or cats.
6. While they do need to be vaccinated, they aren't 'disease and bug ridden'. Most of them are actually very clean, and for every 20,000 cats that get rabies, 10 ferrets do.
7. While their up-front cost is expensive, they seem to be cheaper to keep over-all than a dog or a cat.
8. No matter how bad of day you are having, it is impossible not to smile with them running around excitedly, trying to get you to join in their games.
9. If you have a rodent infestation in your house, it will be no longer. Ferrets are exceptionally good mousers.
Cons:
1. As I am sure you've heard, they DO have a natural musky odor to them. However, the smell actually really doesn't stay on the ferret, and if someone cleans out their cage and litter box every day or every other day, then they hardly smell at all.
2. They are higher maintenance then say, a dog or a cat. They need lots of attention, someone to spend hours a day with them playing and keeping them out of trouble, and they need to have their litter and bedding changed a minimum of five days out of the week.
3. They will never be 100% on litter training like a cat will. They will occasionally have an accident, and if someone can't handle that, this isn't a good pet for them.
4. They get into EVERYTHING. I always say that if beavers are the architects of the animal kingdom, then ferrets are the interior decorators. Cupboards will need child proof locks, you will find them in places that it astounds you they could even reach, they will dig up potted plants, they will "reorganize" their cage, they love stealing phones, remote controls, car keys, pencils, feather dusters, water bottles, and anything else they can get their paws on. Ferret comes from a Latin word that means "little fur thief". This is not a cute little moniker. This is an omen. Putting things up and out of the way of a ferret will become a way of life.
5. You really need to get two or three of them. Most of them don't do well alone.
6. If ANYTHING goes wrong in the handling or training of a kit, you can have a monster on your hands. They need to be nip-trained, played with, and handled for hours on end when they are babies to make sure they are the friendliest they can possibly be.
7. They are illegal in a lot of areas, and there is a heavy fine for anyone caught with them somewhere that they aren't allowed. In California it is a 2,000 dollar fine per ferret.
Over-all, if someone has the right household, then the pros definitely outweigh the cons. They certainly aren't for a lot of people though. They need to be with a very tolerant, responsible, patient owner who is tidy enough to keep up after them but not so much of a neat-freak that they get upset when the ferret decides to demolish a roll of paper towels all over the living room floor. But to be honest, the biggest thing that a ferret owner needs is a sense of humor to deal with these pesky yet adorable little critters.
-----This is something I got from another question asking the same thing. Credit goes to Melissa B! Anyway, this is all true.