My experience is based on a a Wolf car cover that was bought 10 years ago. I don't remember the specific fabric name but it was the best outdoor car cover they sold at that time. Since then here have been significant advances and other companies may have better stuff.
At first I liked it and used it regularly. Then after a few months it became more of a pain in the ass and I did not use all the time. It certainly is no substitute to a garage. Your best protection against sun and rain is probably regular waxings, tinted windows and sunshades. Use only if your primary concern is to keep dust, grit and tree droppings off but keep in mind that any crap on the car get's rubbed in by the cover. Also, alot of the crap that the cover is supposed to keep off the car tends to find it's way to the car anyway. Also, covers screw with fresh wax and then the fresh wax gets in the cover. Which means that if you now cover your spotless car, it is going to get a dusty/waxy residue back on it and ithat crap is really hard to get it out of the cover once it gets in it. Also an outdoor cover needs to be breathable, but don't count on any breathable cover keeping the car dry from heavy rainfall.