Well, seeing how you have 2 cats, you're probably well prepared for a ferret. I have two: a 2+ year old Hobbes (from Calvin and Hobbes, I'm Calvin), and a 1 year old named Y.T. (from the book Snowcrash - Y.T. is albino - whitey, get it?)

Anyhow, ferrets are like kittens in their stupidity and their playfulness, and they stay that way forever, at least until they are too old too move quickly anymore. Even then the spirit is there. They do have some oddities though. They are extremely obsessive / compulsive. Sometimes, such as stealing guest's shoes and hiding them under the dresser, it's really funny. Sometimes when they steal your stuff it's just plain annoying. And they never give up on something either. Once they realize something can be done, no matter what you do to prevent it, they will still try. This is why I have claw marks on my dresser drawers, because they really want to get in there again. Lastly, they really like to dig. Plants, carpeting, you name it. Quite destructive really. My carpet wasn't great to start with, but the next owner of this house will definitely be replacing it.

In spite of it all, I love them. If you like the rest of your pets, I'm sure you'll get along with a ferret just fine. I got two, a second one to keep each other company while I'm gone. You may be fine with one, it will spend most of it's day pestering the other animals.