My boy loves his - he goes a bit over the top actually - it only takes a couple of spots of light rain & he legs it in there. He also goes in when it is too hot, or if he ever injures himself in the field he goes in to sulk!
Hello I have mobile stables on my field and they never used it in winter, in fact i winter my horses on a rented field now with no proper shelter and just take them back in really bad weather. However in summer they use them to get away from flies and heat, despite wearing fly rugs. A lot of people i have spoken to say theirs only use them in the summer as well. Aim to get another field in the future for winter use and put a proper shelter on this though, just for my piece of mind really. I think its good for them to have the choice.