If you have a system with a good front stage and decent dynamics, you'll never miss rear speakers at all. In fact the other day I was playing my system for someone that doesn't know much about audio and he asked if there where speakers in the rear, I said no, and he almost couldn't believe all the sound was just 5 speakers, 4 up front and one sub.

If your system sounds decent, you're not missing anything without rear speakers. My "rear fill" is what ever natural reverberations that take place in the car.