As was touched on earlier, they only have one stereo input from your sound card, so those with a SB Live or other 4 speaker output sound card will not be able to take full advantage of the surround sound. For non AC-3 sounds, the ADA880's simulate a surround sound affect. You will not get true 3D positional surround sound that you would with a 4 speaker system with both front and rear inputs. For DVD's the center channel is merely redirected to the front left and right speakers, and can be adjusted separately from the actual volume. The sound quality is great, and the speakers are really powerful, but the rear speakers are smaller and are rated at less wattage so the surround effect is lost a bit.
If you have a DVD player with AC-3 out, but don't have a front/rear output from your computer, or don't care for positional surround sound in games, then these are a great buy, especially at that price. I originally ordered them with my Dell, but replaced them with the Cambridge Soundworks Desktop Theater 5.1 so I could have positional surround sound as well (and true 5.1 surround sound).