Ferrets are very playful, with each other and with you, and they don't lose much of that playfulness as they get older. A ferret -- or better, two or more -- can be a very entertaining companion. They are smarter than cats and dogs, or at least they act it. They are also very inquisitive and remarkably determined, which is part of their charm but can also be a bit of a bother. They are friendly, and they do know and love you, though for some of them it can take a year or so to fully bond.
Ferrets can be trained to use a litter box and to do tricks, and most of them love to go places with you, riding on a shoulder or in a bag. They sleep a lot, and they don't particularly mind staying in small places (a cage, for instance, or a shoulder bag) temporarily, although they need to run around and play for at least a couple of hours a day. A "single" ferret won't be terribly lonely, although the fun of watching two or three playing together is easily worth the small extra trouble. Barring accidents, ferrets typically live 6-10 years.
Ferrets have lots of good points as pets, but there are some negatives as well. Like kittens and puppies, they require a lot of care and training at first. They're "higher maintenance" than cats; they'll take more of your time and attention. Ferrets have their own distinct scent, which bothers some people, and many of them aren't quite as good about litter pans as cats are. Although most ferrets get along reasonably well with cats and dogs , it's not guaranteed, so if you have large, aggressive pets (particularly dogs of breeds commonly used for hunting), keep that in mind. Likewise, small children and ferrets are both very excitable, and the combination might be too much.