I have the Z-680's and I love them for what they are. They aren't going to give you the sound of a $5,000 home stereo system, but I didn't expect that. They are some of the best 5.1 speakers you can buy for your PC, though. I've seen them for as little as $220 which is a great deal IMO.
The Z-680's are sort of like the Koss PortaPro or KSC-35 in that they have plenty of bass and aren't the most accurate sounding things you've ever heard, but they make music very fun to listen to! And you can always make adjustments to the sound via the control pod or a software EQ.
Another great 5.1 set would be the Klipsch 5.1's or 5.1 Ultra's. Both are very nice but the horn tweeters make the highs shrill-sounding to some. The only thing I don't like about the Klipsch set is its lack of flexibility compared to the Z-680's. The Z-680's come with a Dolby Digital/DTS decoder, as well as optical and coaxial inputs, so you can hook it right up to a DVD player/PS2/XBOX/etc. as well as your computer and watch movies in DTS and Dolby Digital or play games in 5.1 surround.
If you don't plan on playing games or watching DVD movies, there's no real reason to go with 5.1 and you would do just as well with a good 4.1 or even 2.1 system.
Cambridge Soundworks makes some pretty nice 5.1 sets also, but I've had personal experience with the Logitech and Klipsch sets and would recommend either of them, depending on whether you needed the digital decoder or not.