Sorry for the long post but I hope it is beneficial.
A bit of a tale of experience from me. I am super anal about my cars. I imported a 97 Toyota Supra in 2011 from Japan, the car was immaculate with 56K miles.
Won best on stand at JAE 2012. At the time I didn't have a garage and became a little paranoid about the elements eating away at my pride and joy. Came home from work one day to find the car almost covered in bird crap. I decided to get a cover, the Stormforce ones came highly recommended on the forums so I took the plunge. Good quality piece of kit, fleece lining inside but not overly thick, good water resistance outside. Only issue was that the elastic in a slight wind would sometimes lift and ruffle up at the from of the car.
I decided to make an improvement to the design as attached and never had the problem again. Just a simple loop at the front and then bungee cords clipped to the towing eye or front suspension.
Years one to three were relatively uneventful and only really became a problem is high winds and storm conditions as the cover tried to lift off the car. Getting the straps underneath the centre of the car with a broom was also a pain in the arse. On more than one occasion I was out at 3am in the rain taking the thing off as it was all over the place, even with the straps as tight as possible. It seemed that every winter there were light scratches appearing that needed to be polished out, particularly on the front and rear wings where it had been moving up and down with the wind. The car was always cleaned before being put under cover....

Condensation had always been a slight issue but minimal.
Year four is where the problems really began to mount up. The cover seemed to lose its water proof abilities and we started seeing massive amounts of damp under the cover when we removed it. We also saw serious "scratches" appearing all over the car.
A forum member made me an offer for the car that I couldn't refuse but I insisted that I got the scratches in the paint sorted first as I wasn't happy. I took it to a detailer in Preston to give her a once over and polish out the mysterious scratches that kept appearing on a seemingly immaculate and well looked after car. He rang me a couple of hours later and told me that after much effort, the scratches were not coming out, even though they seemed superficial.
He believed that the scratches were indeed cracks in the paint, possible caused by the car continuously being in a moist environment over winter. Even on a dry day, the moisture under the cover remained. I had already stopped using the cover but the damage had already been done...
After getting a second opinion, the car had to get taken down to the bare metal and resprayed on the rood, boot lid and bonnet. For me the car never looked right after that and in certain lights you could see that work had been done.
Strangely enough I would only ever use a cover now in Summer to keep bird crap and tree sap off the car. In winter I think a few coats of wax will do a lot less harm than a cover. Would I have one again, not on your Nelly..!!
H.