Welllllll, this is really a trick question. If we had Mysterious Fai all the time, I give it three chapters probably before I start wanting to get in there, stab him, and tell him to get over it. If we never saw anything but Playful Fai, I'd want to fly over, stab CLAMP, and tell them to make characters that are actually human, cause no one's REALLY that happy all the time. So we NEED both, or he'd completely suck. XD
My opinion. *shrug*