I have three (well, 2 are my room mate's) and we love them dearly, but I can't argue a bit with anything said above. I read somewhere that all ferrets are frustrated interior decorators trapped in the bodies of weasels-yes, they WILL destroy every piece of furniture you own. My Koto, an exceptionally well behaved ferret, made a tunnel through the mattess of my hid-a-bed couch. And packed off a family of rubber duckies, and dug up my umbrella tree, and dug at the carpet in my bedroom, and pooed in my box of printer paper, and stole dog and cat food every chance she got... But she also curled up on my feet each morning while I brushed my hair and came out from where ever she was when I went to bed to snuggle up next to me and sleep under the covers with me (picture a twin bed holding 1 average sized human, 1 85 lb dog, 1 cat, and a ferret, all sleeping soundly... it was cozy!)
The best ferret-keeping tips I can give from my years of experience:
-older rescue ferrets can be sweet (my darling Koto was found, abandoned, in a snowy parking lot)
-if you get a good one who respects the idea that pooing everywhere isnt ok, it will still pick a few favorite corners-not the same you pick- and they will likely to be impossible to put conventional litter boxes in. Koto chose narrow spots behind doors and I became adept at turning disposable aluminum pans (like from lasagna) into makeshift boxes all over the house. Invest in pliers if you don't already own some.
-put a bit of poo in each new box to prevent it from becoming a toy instead of a toilet.
-Never underestimate their ability to get into places they shouldn't be. I have a friend whose ferret TWICE somehow escaped and found its way into the walls of the house. Yes, literally inside the walls. He is a carpenter, and he still can't figure out how the weasel did it. Any time you let them go loose, watch them for at least 10 minutes. They will find at least a dozen things they shouldn't have that you missed when preparing to let them out.
-Finally, get the biggest cage you can fit in your house. If it must stay inside while you work/sleep/have guests, it will be much more tolerable if it is not in the bare minimum space. Yes, they are always asleep when you see them at pet stores, but they are a lot like kittens-when they are awake, they are AWAKE and want to play. Our 3 have a 2 story cage 2' x 3' x 4' with a ramp, 2 long tunnels, 5 differently shaped hammocks, 2 litter boxes, and a den. You don't need all that to start, but if you get a big cage and one hammock you can start to figure out wht YOUR pet wants to play with and gradually add more by purchasing or building. And if you have a pet store which allows you to bring in pets, you can let your ferret pick its own bed and toys.
Be sure you want to commit to it, but if you do, I wish you luck! I love my ferret, but honestly, I don't think I will replace her with another when she passes. They are a lot of pet and I just don't think I need more (like I said, Koto was a rescue-I wanted a hedgehog and I ended up taking her in instead. I would rather have a hedgehog than any other ferret but her!)