The biggest negative with my ferret is I would have to bathe him weekly/biweekly to prevent the smell. He didn't smell terrible, as he was descented, but it was unpleasant if I let him go longer than two weeks without a bath. He was excellent about his baths though and would usually just fall asleep in the little tub I used.
I got my ferret when he was a baby and held him ALL the time. He was very, very friendly and never nipped or bit. He would play bite when I had him out but it was not hard at all. He was very sweet, but not all ferrets are. If they are not handled as youngsters or depending on their personality, they may bite hard and draw blood.
The saddest negative is the fact that pet store ferrets are very prone to cancer and that is what I lost mine to. I only had him for five years before he developed lymphnotic cancer. It can be difficult to find a vet to treat them because most consider ferrets exotic. It was also difficult to put him to sleep because they had to access his circulatory system directly through his heart due to insufficient size of veins in ferrets' legs. If you decide to buy a ferret, be sure to buy one from a breeder. Also, it is very, very smart to have them descented and neuter/spay can help with their personality as well. They are not usually cheap pets, but they can be wonderful if you have te time for them.
Another thing I thought of: It took about a week to litter train my ferret. They aren't naturally prone to using them like most cats. To train him, I had to sit by the cage and watch to see if he looked like he was going to go to the bathroom. If he did, I would scoop him up and put him in the litter box and wait until he did his thing. After a few times of doing this, he caught on. It was just a matter of catching him before he went to the bathroom outside of the box.
