"Some dream!" doesn't sound sceptical. It sounds as if you are either very impressed with the content of their dream ie "Wow, that was some dream!" or rather disappointed "Hah! Call that a dream? That's rubbish".
It's not old-fashioned either.
I don't know what you mean by being sceptical about someone's dream. Do you mean that you don't believe that they actually had the dream they have described to you?