I've never owned a ferret myself, but I've cared for them quite a bit in stores and at the zoo (educational animals). Here's what I would say:

The pros would be how playful and smart the little buggers are. They are so cute to watch and fun to interact with. Just be sure to get them young so you can train them out of the bitting stage, or get an adult already trained. Either way get them from a trusted source.
Cons would be that you have to bathe them at least once a week or get the scent glands removed (unnecessary but lots of people look into it) especially with males. They are the consummate escape artist, and can get into ANYTHING so you have to ferret-proof your entire home, rather like having a destructive toddler. Also, you need the biggest cage you can afford, or you have to take them out for exercise and play time quite a bit. They need companionship and attention.

The clothing...it's just like the clothes for dogs: totally unnecessary. Mostly it's made for girls who want to dress up their pet all cute. Leave that stuff on too long and it will be shredded in no time. Plus the difficulty in getting it on them in the first place...it's not for nothing my BF calls them furry slinkys.

No, those harnesses should not be left on all the time. Heck they shouldn't really be left on a dog ALL the time, but I know it's for training purposes. Collars, harnesses, and other such things are put-on-when-needed, take-off-when-not items. Since ferrets can compress themselves to fit into the smallest spaces, if they have something around them all the time they could get it twisted and choke. Ferrets are so much like little babies or toddlers it's not funny.

Hope that helps.