Sonos devices use a wireless mesh network to carry music throughout your house, typically configuring a single endpoint--which Sonos calls a ZonePlayer--up as in a separate room. You can connect ZonePlayers to your existing home theater speakers, bookshelf speakers, or fancy wall- or ceiling-mounted speakers, whichever makes sense in each room. Using a PC, your iPhone, or Sonos' custom remote you can configure the system to play different tracks in each room, or synchronize audio across the entire house. The end result is a music streaming solution that's as dead simple to use as an iPod.
Like the other ZonePlayers, connecting the S5 to other Sonos devices is as easy as pressing the connect button on both devices. Also like other ZonePlayers, the S5 includes a pair of Ethernet ports, which allow you to hardwire it into your network and still use the port for another device. While each Sonos device can connect to other Sonos devices wirelessly, you'll need to connect at least one ZonePlayer (or a Zonebridge, which serves the same purpose) to your existing wired network. The Sonos gear won't pull music off of your existing Wi-Fi network. This isn't a problem if you've already invested in hardware, but if you haven't and want the S5 to operate wirelessly, you'll need to buy a Zonebridge for $100. If you don't have an iPhone, iPod Touch, or iPad and would prefer not to use your PC or Mac to control the S5, you'll need to buy a remote from Sonos as well, which costs a spendy $350. The The S5 also includes an audio input, which you can use to stream audio from an external device through the rest of your house. I didn't find this feature particularly useful, but your mileage may vary.
The S5 is a great addition to the Sonos product line. However, if you haven't already bought into the Sonos vision of total-home music, it may not be the best fit. There are cheaper, entry-level streaming solutions out there, especially if you're not planning on adding more rooms to your system later. However, if you want to add a central stereo to your home, the S5 is a great way to get started, or add a little portable kick to your existing Sonos system.