Amount of time is not directly proportional to the amount of fun to me though... I figured out the chess ones and the weight ones in relative short order, and loved them. It took me FOREVER to get the dog hit by a car one b/c I thought I had the solution. What I had done was take two of the legs to the top, but didn't rotate them correctly - I mean it just got ran over by a car! What difference does the rotation make! The game didn't like that as an answer, so I wasn't quite sure what it wanted. I HATED that puzzle.
I think what makes a good puzzle to a particular person is how that puzzle engages your mind as your trying to solve it.