It is pure brilliance!
The flip not only signifies that what you are about to hear may be a little twisted, it also puts The Joker visually mirroring Batman so that scene cuts and transitions are not as jarring and the viewer can focus more on the dialogue rather than the visuals.
In my opinion The Dark Knight is one of the best movies I have ever seen. Nolan's portrayal of Batman is probably the truest version of the character I have ever experienced so that alone wins me over... but I thought the film itself, the storytelling, and messages within it made for an overall excellent piece of cinematic art.
So, obviously I'm PUMPED for the third one.
Can the excellence of the second film be surpassed by the stories conclusion in the third film? I doubt it. But I'm sure it is going to be a blast regardless!