Hi Rocheal! To be honest I can't think of many downs... BUT several ups, IMO.
Ferrets are VERY smart animals and learn VERY quickly, although they are stubborn & persistant sometimes. I don't know anything about flying squirrels so I really can't compare there.
Ferrets love attention! They more you give them, the happier they are. There are some things that you really need to be aware of though... They LOVE rubber, why I do not know but they do. The soles of your shoes, the buttons on a remote control, bouncing balls, you name it... if it's made of rubber or anything similar they will most likely chew on it & play with it... try to avoid allowing this as the rubber won't digest & could actually cause a blockage in their intestines, which could be very dangerous & possibly (worse case scenario) even require surgery. I give my ferrets the stuff called ferretlax, which is a flavored gel like ointment in a tube that you feed to them periodlically. Although I have heard many other ferret owners say vasoline works just as good. It helps to lubricate the digestive tract to avoid blockages & hair balls.
Another thing is (according to some reading I have done on the web) ferrets supposedly have a short term memory and if out of the cage roaming will not remember to go back to the cage to go potty if they are too far away from it. Personally I don't agree with this as all of mine go back to their cage as long as I leave the door open. But maybe I am just lucky? Thought I'd mention it anyway... you may want to invest in some litter boxes to place in various corners just in case. As a general rule, they almost always go in a corner.
Ferrets are creative creatures and also climbers. If you don't want your ferret to steal your keys, wallet or other small objects keep them up high where you know there is no way for a ferret to get to them or else keep them in another room the ferrets can't get to. Also, in this catagory, if you have plants you may want to put a thick layer of rock on the surface of the dirt so the ferret doesn't dig them up because the love to dig & they love dirt!!
Having a ferret is like having a 2 year old child running around all the time... they are always into something & need to be watched carefully so they don't get into anything that can harm them.
It is very important to ferret proof your home or the area that they will be allowed to free roam. This means even getting on your hands & knees & finding ever nook & cranny there is that they may be able to squeeze into. (behind the stove, under the fridge, inside the base of the cubbords, under or behind the washer or dryer, etc) If their head will fit the rest of their body will too!
You should do some research on ferret health as well to learn what is normal & abnormal in daily ferret life. As kits (youngins) you are safe for the most part, but as they get older, like humans, the start to develop health problems. Granted not all ferrets have health problems, but it's nice to know about anyway. One of the most common is Adrenal Disease (A big sign of this is hair loss starting on the tail.) There is also Insolinoma, Diabetes, & Deafness. (although they can be deaf at any age, that is more of a hereditary trait most common in pandas, blazes & DEWs or dark eyes whites) You deffinately want to make sure there is an exotic vet with knowledge of ferrets in your area.
Personally I think ferrets are a great pet, but then again I am biased (I have 4 of them). I would suggest considering 2 though to be honest just cause ferrets love to play & have lots of energy. That way they don't get lonly while in their cage or while you're away.
There is actually A LOT more to learn than what I just said, so if you have specific questions please feel free to ask!! I'm here for ya no matter what you decide!