They are indeed an exquisite cookie. Think of a rectangular Oreo, with chocolate cream filling that is coated in chocolate. Mhhhh! The combination of the chocolate cookie, the cream and the outer shell is fantastic. I've had a chocolate covered Oreo and the Tim Tam win hands down. Plus there is a benefit to not only the coating but also the shape.
The rectangular shape allows you to do a "Tim Tam Slam." This is accomplished by biting off opposite corners of the Tim Tam and then using it as a makeshift straw for sucking coffee or tea. As you dip one corner into the hot beverage you suck on the other corner until the liquid begins to dribble into your mouth. After a few sips the structural integrity of the cookie will begin to become compromised, this is your cue to shove the whole cookie in your mouth.
You've gotta love the Aussies!