I've gotten the center speaker, and it makes a huge difference when I watch TV or movies. Dialogue is much more pronounced and doesn't feel far away.
My receiver is a 5.1 Pioneer VSX 819H, so I have the option of adding a subwoofer or rear speakers later. The cable box hooks up to the receiver with a HDMI cord (video and sound, not just pass-through). What's confusing is that there are about three stereo sound settings and a dozen surround sound settings, so I'm doing a little trial and error to see what sounds best. It's true that, with some settings, the center speaker becomes fairly useless (as do the front speakers), but I think I've found the right setting to take advantage of the center and front sound.
When I'm listening to CDs, I set the receiver to stereo. I can set it up to use the center speaker, but I prefer the stereo sound for music. I'm guessing most in this forum would agree.
My next step, somewhere down the line, is a decent subwoofer. I've read the above replies, and I'm sure it would make a difference. But since I'm not usually blasting music (I have a one-year-old, with another on the way), how big of a difference do you think adding one might make?
One thing I think I've noticed (and perhaps this is for another thread) is that with music made within the last ten years or so (or, at least music that appears to have been made with a serious production budget), the bass does appear to be slightly weaker. When I listen to older music, I don't seem to have this issue as much. Sure, I'm making a pretty gross generalization, but does anyone else think that more contemporary music has been made to take advantage of subwoofers?