I recently added a center channel. My system had 4 speakers and no sub for about two years, before this latest addition. Adding a center speaker made a huge difference.

Certainly for DVD's it has certainly made the dialog substantially more clear. For multi-channel music like SACDs, my impression is that the addition of the center has improved the performance of the other speakers. For example, eventhough the center is in front, the rear speakers seem more alive and present than before. I know this sounds strange, but this is what I perceive.

Also, stereo music made into 5.1 channels through Dolby Pro Logic processing sounds much better than before. I still prefer listening in plain stereo for most two-channel sources, but for some types of music, e.g. Ambient music or random listening on the radio, I think it can enhance the experience.

Finally, the center speaker I added was from a different manufacturer than my fronts. I don't think that the center was much different in character or timbre from the front speakers, but still, I noticed a difference. I have since adjusted the center channel EQ through my receiver, and now feel that all the speakers are well matched.