Most everything has been covered by previous posters, but I figured I'd throw this in. As far as them being aggressive, I beleive it comes down to how much they are socialized as a kit.

A lot of them times when you go pick up a pet store ferret that has spent the first few months of its life in a cage with little to no interaction, they will nip quite a bit harder than a well socialized baby.

A truly aggressive ferret is quite rare, however I did have one that bit my finger straight down to the bone on the first night I had her home, and seemed to hate me from that moment on.

My other boy, Xander, that I raised from a kit, was the sweetest thing on four legs, just an absolute doll. As are most ferrets. He did have one strange quirk, though, if I dared squeak a squeaky toy in his presence he would come running up and bite the absolute crap out of me. I guess the sound really annoyed him.