I just bought Z-640 and installed about an hour ago (could've order them online, but bought locally since didn't wanna wait). I think they are pretty good, kinda what I expected - no miracle, but decent sound... I like their design over Creative 6600's (I didn't like the corded remote- looked flimsy), and also wouldn't have really the place for the rear/center satellite, already now I have it crowded), which were the same price ($99).... my soundcard doesn't support 6.1. so I went with this 5.1 setup and don't regret.
On the other hand, if I'd have money to burn, I'd get some of those Klipsch sets; was looking (not listening, but bet they deliver) at them in the store, neat stuff
Z-640 seems to have everything right, and feels sturdier then Creative and looks better as well; soundwise they seem to be about equal (from what I could compare in the store); the Creative sound may have been a bit more overall balanced, but that's subjective observation....
I also noticed Logitech has rebate for Z-640 right now, so I'll print it out and send as well.
Good luck with your pick.
dave