I tried the Mac Jr. It was really good... like a Big Mac, only smaller. There is one patty instead of two and there is no extra bread in the middle.
My knee-jerk reaction is that it is actually better than the Big Mac. I never really thought about how much I don't need that bread in the middle. The Mac Jr. has all the taste elements. It's a more reasonable portion and actually holds up better than the Big Mac which can turn pretty messy. Get a Mac Jr. while you can!