We purchased an Adco cover for our 5er from Camping World in 2009.
While I was skeptical about how it would last it has lasted siince the winter of 2009//2010.
It is relatively easy to install and remove. Our winters are anywhere from 5 degrees Celsius above to 15 below with anywhere upwards of 3 feet of snow average.
The cover has performed well and is still in prime shape.
The big bonus is that it protects the 5er from the harsh winter elements, snow, sleet, sun, etc.
If you don't have covered or inside winter storage a cover is well worth the cost and effort IMHO.