I know DivX contains spyware and crap like that. But I'm with the Pro version and this one doesn't conataint At least i haven't noticed it yet.
I know that XviD is not very well optimized, but as far as i know DivX is optimized for Intel and especially Hyper Threading, and my CPU is AMD Sempron 2200+. Those optimizations are worthless.
FFDSHOW - NO!
Well, i see many people recommending them. I tried them, seem to had problems. I can't remember what kind of problems exactly but i recal that when i went back to the regular everything was ok.