I tried it once, but I don't get the appeal. I didn't really like the Big Mac sauce (though I'm not keen on mayonnaise in anything), and I didn't get the point of putting that layer of cheap bread between the two burgers (it just makes the burger taller without adding to the taste or nutrition).  I've been to McDonalds plenty of times since, but never ordered the Big Mac again. If I'm going to eat something unhealthy, I had better enjoy it.