I used them for several years on several cars with general success. A few suggestions; do not ever cover a dirty car. The car must be perfectly clean. Otherwise the car cover becomes sandpaper. If you do cover a dirty car, then after you polish out all of the scratches, throw away the car cover.
Do not leave a covered car in an area where it will get exposed to wind. I am lucky in that my back yard is quite well protected from wind.
A downside of covering a car is that even if you have a high quality cover (I used NOAH covers), some moisture is retained. Even if your car is completely dry when covered, moisture from rain or snow (even moist air) will cause some rust on your rotors and other parts. This was more noticeable on my covered GM cars than my covered Ford (yeah that will get me flamed, I'm sure).
IMO if you can follow the rules above, covering is better than leaving it uncovered...but it's not a substitute for a garage. I no longer use covers because I built a 3 car garage :yup: