I have 4 of them. I've had 3 of them for 4 years. So I'd consider myself to know more than most about ferrets.
They are smart wonderful VERY high-maintanence creatures. If you're pondering getting one PLEASE ponder getting 2 instead. They are social animals and unless you're going to be there all the time with him/her, give him/her someone to amuse them the rest of the time. Otherwise they can get unusually agressive and/or destructive out of boredom. 2 of mine bite from time to time but they were both abused ferrets from rescue shelters.
I love my critters. They live in a custom-build 3 level oak hutch with french doors and ramps between levels. Most of the ferrets in pet stores are babies so keep in mind that they get bigger. A male can get up to 7 pounds-- a female upwards of 5. Most of mine are 4 pounds right now (they gain 1/3 of their body weight in the winter and lose it in the summer....they also shed in the summer).
My ferrets can open sliding doors, move random laundry etc around to build "ladder" sorts of things to get up higher, and they open cabinets and drawers. They plot. I always say I wouldn't be surprised to come home to find them building nuclear weapons. Mine can open human-baby cabinet and drawer latches so I have to have heavy boxes all over my kitchen (or they get in the cupboards, climb up the back into the drawers, open the drawers from the inside, and get on the counters). One of my ferrets is also a panty/bra thief. Another is a shoe destroyer. Another prefers to hide socks. Yet another likes to steal and hide keys.
No ferret will ever be 100% litter box trained though. They just don't give a damn about pleasing you...unless you have something they want. They only go to the bathroom in corners (if they go elsewhere it's a sign of fright or illness). You don't get to choose the ideal "poop corners" either. They do. Then you accomodate them with a litter box. Depending on the ferret they will use the box about 50-90% of the time. One of mine won't go in a dirty litter box. One of mine digs all the litter out if the litter is too clean. When you have a ferret you sort of realize that you're the "bitch" of the relationship. They rule my life.
I always think I sound negative when advising people about ferrets but it's only because they are a hell of a lot more work than I thought they'd be. If you aren't planning to clean well every day, your house will smell like hell. If you end up getting a couple (and I beg that if you want ferrets that you get more than one) PM me with any questions.
Oh, and they need regular shots. They can get many diseases cats and dogs can get. They are also one of the only pets that carry the same sicknesses we do. So you can give your ferret a cold or flu and they can give them to you. Ferrets are also prone to a lot of expensive and/or terminal medical problems. A lot of that is because Marshall Farms (the biggest distributer of ferrets in the US) pays/neuters then WAY too young. It stunts their growth and increases chances of medical issues. They're also sensitive to plastic water containers and having the lights on at night. One of mine had adrenal disease a few years ago (unusually young to get that) and had to have a $700 surgery. She lost all of her hair. She'd have died without it.