Well I thought of some other things that could be downs. And I also read my poor spelling from earlier and please excuse that I should really proof read!!!
Anyway what I meant to say is that there really isn't any downs but alot of ups, of course I own one and I think that anyone who owns a pet is going to have lots of good things to say.
They require a large cage at the least it should be 2ftx 2ft.x2ft. but bigger would be better. That is their home and at night while your sleeping they need to have room to glide around and play. That is another thing, they are nocturnal and I have been staying up alot later now that I have Owen, just when I get ready to go to bed is when he is lots of fun.
Althought the caging can be expensive, I found a parrot cage on E-Bay for cheap. A parrot cage will work as long as the spacing between the bars is no bigger than 1/2" at the biggest. Now if you know anything about parrot cages your biggest challenge will be finding a parrot cage that is big enough with bars spaced that small together.
You can also buy a cage made for sugargliders but IMO they aren't big enough. And they usually have different floors. A cage for a Flying Squirrel should be completely open, with no added floors.
You can also make your own, which knowing now what I know I probably would have just made mine out of the 1/2" mesh wire and made it to be the size I needed.
Also find somewhere that sells Manzanita Branches (I use Dr.Foster& Smith). There is really no safe pet store toy, so provide alot of branches and ropes and things like that. Try to stay away from any plastic in their cage it can be harmful if they ingest it, a plastic wheel will be okay but check regularly for chew marks.
I just went with manzanita because alot of wood is toxic to them(every friut tree is toxic except for apple, and also watch out for pecan wood as it is also toxic,infact everything on a pecan tree is toxic to a flying squirrel except for the meat of the nut) and they are also very sensitive to pesticides so don't just go out back and chop down some branches. With that being said, if you can buy organic fruit and veggies than do but still wash them. If you can't afford the organics(like me) then just make sure and cut the peel or rind off especially if waxed.
I think that's all for now but I'll probably think of more later!!