My experience was that sync reliability was a function of distance from the screen, at least while I was using the inbuilt 3D sync emitter. I could get reliable sync at about 11ft from the screen but at 16ft, sync would drop out from time to time, enough to completely ruin the 3D experience. Even at the front row, sync would drop out when I turned my head too much away from the screen, although it recovered so quickly as not to be a real issue.
Sync reliability is also a function of your room reflectiveness. My projector is mounted above the screen, and I'm pretty sure that having the emitter in line with screen, as opposed to the (black) wall above it would improve reliability. I did experiment with placing 2 sheets of tin-foil just above my screen (same height as the built-in emitter), each piece about 20" in length. Sure enough I was now able to get reliable sync at my back row. The foil was not visible once the light were turned down for movie watching but obviously they spoil the decor when the lights are turned up all the way. Adding an external IR emitter solved my problem in a more elegant fashion. The external emitter is FAR stronger than the built in unit. i was able to get rock solid sync from 25 ft back and was able to remove the foil sheets. I can even turn my head away from the screen and have the sync persist. I didn't like having to spend the extra $, but I'm very happy with the results.