Hi Lucky,
As Scotch Lad says, there are mixed views and experiences about covers. I am also in the positive camp.
We have a lime tree overhanging where we park the van and the resulting sticky mess each year is a nightmare if the van isn't covered. So we have a long history of covers - almost 20 years on and off - on four vans.
The modern versions from the more up-market suppliers - say, Protec and Specialised Accesories for example - are pretty good in my experience. They fit well and do a good job. I can put the cover on our van, on my own, in about 20 minutes. Most of that time is spent rolling the straps under the van and clipping them on. The only time I need a hand from Sue is if there is more than a light breeze, tending to turn the cover into a kite!
We have never experienced any rub marks, condensation, mould or mildew - though others certainly have. It is a joy that the van comes out from under the cover as clean as it was when you put it away. It is also possible that the van is less desirable to theives because of the hassle in getting the cover off. Having said that, I can get the cover off, irrespective of the weather, in 10 minutes.
On the less positive side, I remain unconvinced that the green "breathable" fabric used by Protec et al is very long lasting. The longest we have kept one is five years and by that time it seems to be getting very brittle and is starting to tear at the corners quite easily. That hasn't so far been a problem to us as the deterioration with each one has coicided naturally with the time we wanted to move on from that particular van, but it has given me pause for thought about value for money for anybody wanting to use a cover longer. They have cost about