I'd say try to find places where you can listen to these systems, although the problem is most of the time they aren't set up properly, especially if your in a BestBuy or Circuit City. I swear those morons couldn't hook up a VCR correctly.
Anyway, try to listen to them because some people like them, some like the sound of the Logitech Z-680's better. I personally think the Klipsch sound seems muffled, and prefer the Logitech's, but this is where your personal opinion counts.
Here are some reviews you can read over from NewEgg.com, but I wouldn't base your decision primarily on reviews, I'd still base it primarily on your own opinion of the speakers.
<b><A HREF="http://secure.newegg.com/app/CustratingReview.asp?DEPA=0&item=36-119-106" target="_new">Klipsch ProMedia ultra 5.1</A></b>
<b><A HREF="http://secure.newegg.com/app/CustratingReview.asp?DEPA=0&item=36-121-105" target="_new">Logitech Z-680 5.1</A></b>
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