What Sonos is - and isn't
Taking the latter first - Sonos isn't a portable solution in that one can't take it out of the house. None of the units are battery powered, for starters, and Sonos can only work on a foundation of a stable home wifi network. For a cheap portable solution, bluetooth speakers are a better way to go. Range and quality limitations, but they do the job.
Using a computer analogy, Sonos also is more like Apple than PC. It isn't a open system, and there isn't much of geek tweaking things about it. And like Apple, this cuts both ways - it works out of the box and one doesn't need a lot of technical savvy to use it.
Although Sonos don't say it as loudly and clearly as they should, at least one Sonos unit needs to be ethernet wire connected to the wifi router.
Sonos is more reliable than any other wireless audio solution I know, but it isn't as reliable as a legacy wired audio system. Uptime for me has been more than 99% in almost three years, but there are times when some troubleshooting can be called for. But the company supported resources to get help to restore the system are also better than that of anyone else in the space, speaking again from personal experience.
What it is: A modular wireless home audio streaming solution with the flexibility to choose units based on specific needs, able to do duty as a CD quality source to a high end audio system, down to small standalone units that don't need anything else to play music. And one can start small and add units over time - in a priorities, budgets and needs based manner. That said, a common saying in the Sonos forum is be careful, the things seem to breed like rabbits, once the first one comes in.
While any one Sonos unit can do this duty if wired to the router, convenience usually dictates that this duty is assigned to the cheapest Sonos unit sold, the Bridge. All this needs is mains power and an ethernet wire running to the LAN port on the router. What this unit then does, apart from pulling music from the router, is to set up Sonosnet - a proprietary dedicated to audio parallel wifi network in the home. This can be affected by the same interferences that affect home wifi operation, but it is immune to what is happening in terms of loading on the home wifi network. Once the Bridge is installed, other Sonos units need only mains power, getting music signals wirelessly from the Bridge, and if necessary, from each other - all work in peer to peer mode when called to do so, and are not architecturally constrained to the hub and spoke model as Airplay.
The usual way to get owned music to play via Sonos is to have it contained on a HDD that is wired to another LAN port of the router, or to the LAN port of the Bridge. Every other Sonos unit can then wirelessly pull the music for replay in the room where that unit is located. It can also pull the music from the net, available to the router from the broadband socket to which the router is connected.
For home audio one can choose between the following units for different rooms, based on what currently exists there, or is planned to be installed there.
1. Connect: wired to the digital or analog inputs of any DAC or stereo amp or active speakers, provides music from the HDD or the net, in the room where the system speakers are installed. If the files are lossless, heard sound quality is as good as CD play.
2. Connect Amp: wired to passive speakers that have no amplification built in, does the same thing as the Connect.
3. Sonos play units - 1, 3 and 5: All in one units that contain amps and speakers, and don't need anything other than mains power to pull and play music as above. And two like models can be set up as a stereo pair, where the left right channels are separately fed to the speakers.
4. Sonos Sub - subwoofer duties
5. Playbar - largely a HT solution
The entire system is controlled from a free Sonos app that can be hosted on PC, Mac, Handheld apple and android devices as well as tablets. Using it, one can choose to play in any room, or some, or all together, in perfect sync. Or, one can choose to play, for example, one album in one room, another in another, internet radio in a third, at the same time. Up to 32 rooms! One can have as many controllers as one has devices, all operate independently, all are in master mode.
Set up is very simple. Takes more time to describe than do. Indeed even unboxing the product takes more time.
As I wrote, uptime isn't 100%, a situation inherent in the state of wireless tech today. It is also digital - if it works it will work to 100% of designed quality. If it doesn't it won't deliver any music, no halfway houses.
But this isn't a worry because the support is world class. Free lifetime support from Sonos who are able to remotely diagnose the system, since it is connected to the net. And it works, all my support has been via efficient email, given the time difference with the US. And more often that not, the Sonos user forum has enough people that are knowledgeable and helpful at such times. I am a Sonos fan because of this support more than anything else.
Sourcing: I bought my first set of units from Amazon US, and paid the listed price plus 20% for duty and freight, up front, to Amazon. All units shipped from the US, landed at home safe and sound, in 10 days. Later Amazon stopped India shipping, and I had noticed that Sonos kit was available in India for silly prices compared to those I had paid. I noticed recently that India shipping has again started, and this month I moved my entire system to an all Sonos platform. Same efficient delivery. Warranty support could be an issue, but I have had no failures since August 2011.
. Another ZP90 + bridge is going to offset me by 20K+. 
