Sonos Play:5 All-In-One Wireless Music Player With 5 Integrated Speakers have the ease and flexibility are equally impressive. Each zone can play any independent track or other music choice, or be instantly grouped (synchronized) with any number of other zones. Even when zones are synchronized, it's a snap to control the volume of any individual zone within the group or the volume of the entire group, while maintaining the relative levels you have previously set. The ability to simultaneously play different tracks in different rooms from a single iTunes library attached to a single computer (or from multiple libraries on multiple computers) is a pretty impressive technological feat. (This can not be done with a standard AirPlay device.) Any or all of those zones can be beautifully controlled (from wherever you are in your home) by a single iPhone, iPod touch, iPad, android or by any number of such controllers.
If you also have the audio from an Apple Airport Express linked to a sonos line-in so that your friends can stream music to your sonos system, directly from their iPhone (using Apple Airplay) from anywhere in your house. That works well, but sometimes a streaming iPhone will loose its WiFi connection as you move about the house. (Of course, iPhone/Airplay WiFi streaming glitches are not sonos related problems. This is more of an illustration of how a WiFi based streaming system is likely to work in general.) Nonetheless, it's great to have the airplay available for friends.
