The easiest way I can put it is it's like having two year olds running around. Whatever they can get into, they will get into, whether you like it or not. Ferret proofing is an ongoing, constant process. You'll never wear matching socks again.
They are a LOT of hard work...can't emphasize that enough. They need constant attention and play. They're not an animal that you can cage for long periods of time, so if you have a really busy lifestyle, are away a lot, or can't dedicate a few hours EVERY day to them, they're not for you.
They are very expensive animals, as well. They are very prone to cancers and illnesses that require specialised exotics vet care and those bills add up quickly. Expect to spend, on average, around 8k, per ferret over their lifetime (6-9 years)
I wouldn't change them for the world though. They are the most hands on animal I've ever known, but they are funny, affectionate, cute and so worth the hard work.
Just research, research, research. They are tough little buggers, but so much can go wrong with them, they really need dedicated owners.