I think the size of the house makes a big difference on if a theater adds value or not.
Essentially, is the buyer "sacrificing" space to have the theater? Do they give up a multifunction game-room or a bedroom for a third child?
In my house, I believe my HT would add value to the home. My house is around 3600 sq ft (not a mansion by any means). It has 4 bedrooms, a formal dining room, a family room, a game room, a breakfast area off the kitchen, three and a half baths, a study (which is a bar room for us) and in the backyard is a kids "playhouse" with electricity and a/c. My small theater on the second level is about 9' x 16' with a 11' x 6' computer nook off of it. It was an unfinished attic space that I converted. Since a typical buyer wouldn't be necessarily giving anything up, and since my neighborhood is geared towards families, I think it would be a bones with value. But, that's just my opinion, and I don't believe it carries across to all homes.