Using the same formula as for the Z-680, the 5500 goes beyond being merely a speaker system. It adds an essential component: the decoder, which also serves as a preamplifier that can make audio adjustments and also select from among several sound sources. It's accompanied by the indispensable infrared remote control.
With a system that offers so many capabilities, you don't necessarily stay in front of your computer monitor all the time, and being able to access the various adjustments remotely is necessary for a minimum amount of user comfort. Especially since there are quite a few possible adjustments to make on a 5.1 system!
You might wonder about the fact that Logitech hasn't given in to the pressure to multiply the number of speakers by moving to a 6.1, or more likely a 7.1 system. But, as we've already had occasion to write, it has to be admitted that most sources offer a maximum of 6 channels (5.1). Few DVD movies have moved to 6.1 and, even with the ones that have, you also have to admit that the improvement made by the additional speakers remains relatively slight. Also for games, the improvement a 7.1 system can make will generally be marginal. So Logitech's choice is a reasonable one, since 5.1 remains the best compromise between audio capabilities, installation constraints... and of course price!
As is usual for all loudspeaker systems for PCs, the 5500 comprises a subwoofer enclosure - also containing the power supply and power amplification - five satellites, and the decoder (Control Center), which connects to the subwoofer unit (via a lockable DB9 connector) to supervise the system as a whole. The esthetics, compared to the preceding version, have been reworked and made more traditional. Its combination of black and metallic finish should please most people. On the other hand, the subwoofer has gotten bigger, which won't necessarily be appreciated - but it's a simple fact that good bass reproduction and reduced size are difficult to reconcile. Logitech has decided not to stint on resources here, and it would be inappropriate to criticize them for it!