I had a room where a diagonal orientation was the only option. It was a decently sized rectangular family room, but the short wall where the speakers would ideally be placed had a fireplace in the right corner. Neither of the long walls lent themselves to orient the system that way, and the back wall, the other short dimension, had open stairwells and doors, so that wasn't an option.
So diagonal was it, with the system shifted to the left front corner area. Seating was arranged such that the boob tube and fireplace could both be enjoyed. The couch was diagonally oriented, parallel to the plane of the speakers. The left and right speakers were far enough out of the corners to be spaced wider than the couch, resulting in a perfect equilateral triangle with the center seat of the couch.
It ended up working fairly well, better than expected to be honest. The most significant difference from a typical longitudinal orientation was a distinct reduction of hard, early reflections, resulting in more direct to reflected sound. With more "source" and less "room" hitting the ears, it imaged rather well.
