They work great when the wind doesn't blow. Otherwise the damage caused to the paint from rubbing and and scratching isn't worth it. Never covered anything we've owned.
Seen those that choose to do so, as a matter of fact just tonight. Many new covers torn and flapping in the breeze here in MI. Holes torn in the covers everywhere.
I'd bet there is going to be some damage to quite a few rigs in the storage yard and possible roof damage due to the covers.
Nobody has convince me they are worth the trouble. Just keep it waxed and clean.