We were planning to transition our dog from our bed to his own bed and decided to give this one a try.  From the moment I took it out of the box, he climbed in and curled up.  He has been sleeping in it every night since then.  We bought a medium; he's a dachshund/terrier mix, approximately 30-35 lbs.  He fits nicely in the bed and seems to like that the bed is not too big so it cradles him while he sleeps.