I think in addition to the film differences that Brian mentioned, it also can depend greatly on your physical setup for the surround back channel[s]. I have a somewhat unusual arrangement, not in having two surround back speakers (both the same signal), but in that they are aimed at the wall about 10' behind me. That is, the wall is about 10' behind me, the speakers are right behind my couch. This makes them quite hard to localize in terms of left to right, but quite easy in terms of being behind me. They're spaced about 5' apart. I had 5.1 for about five years before moving to 6.1 with the 7.1 speaker setup. I haven't found a movie yet that I think sounds worse with 6.1 processing.
Aaron Gilbert