We have them for camping, and they are great (although they melt if you take them round the fire and a spark flies, as DD discovered unfortunately).
However, I don't know if we have an unusually small washing machine, but they don't fit properly into ours with the result they don't move about enough to wash properly. I think ours are king size so maybe double would be ok, but I am reluctantly planning a trip to the laundrette, which is more of a pain than changing the cover.