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I have had my Z-680 set since June last year and I love them. They are quite powerful, and have very good quality for a PC multi-media set. Although my experience with my set has been nothing but bliss, let me tell you of a couple of common issues I've heard of with these that you may run into. First, if the room these will be in has low ambient noise, i.e., the computer is quiet, and there just isn't much background noise going on, you may notice some hiss coming from the center speaker when no sounds are being played. They have been trying to fix this issue through various firmware updates to the control pod, but some units still have the problem. Second, the bass from the subwoofer can be a bit overwhelming (mostly referred to as "boomy") at the default setting. You can easily lower the volume of the subwoofer to make this less apparent but some folks still think it's too boomy at the lowest setting. This definitely depends on personal taste.
Other than these issues, there isn't much to worry about. For boominess, it would be a good idea to listen to a demo setup at a retail store, but you may not be able to assess the hissing issue because of the ambient noise that's usual with B&M stores. The most comparable systems to the 680's in terms of overall power and quality are the Creative MegaWorks 550D and Klipsch ProMedia 5.1 just to give you some more choices. These sets may offer slightly better musical quality if you do more of such than gaming/DVD listening they might be better. They don't have the digital capability and Dolby ProLogic II that the 680's do though. I don't think you can lose with any of these.
Lastly, you can find links to web site as well as user reviews
at Neoseeker (caution: couple of popups here: have your popup killer ready
).
edit- fixed Neoseeker linkage
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