Hi Andrew
We bought a Pro-Tec cover three years ago. The first year it was great and lovely to take it off to a clean caravan in the Spring. The second winter when removing, the van was covered in shiny and dull patches all over, a bit like a leopards skin effect. More or less polished off but still slightly visible. This year all was ok but there are one or two points to consider.
Firstly they are not very easy to put on, especially on a windy day. We use two poles which have tennis balls stuck onto them and place one in each front corner and 'walk' the cover over towards the back of the caravan. This is easier with two taller people.
Secondly, if you have any awning rail ends which stick out and are sharp, cover them with something like a cork or similar, as we did incur two small holes from this.
Covers have their uses but I personally find them a bit difficult to use and they would not be my first necessary bit of kit. Conversely, when the weather is bad there is a measure of reassurance knowing the caravan is covered, so generally it is somewhat difficult to give precise opinions.
Hope this helps!!!
Chris