I really want to recommend Sonos, but after a year of ownership, I'd recommend a traditional amp and speakers. Mostly because the Sonos ecosystem is too restrictive, their controller app is God awful, and sadly Plex gave up on the Sonos + Plex integration almost immediately after introducing the feature, so your mileage will definitely vary if you're looking for a Plex companion in Sonos.
I've actually had a better experience since I bought a Chromecast Audio to plug into my Play:5 and began casting from within the Plex Android app ( the Sonos speakers will show up as a "Flingable" device in Plex, but you can't actually fling to them. You'll need a Chromecast Audio for that).
The problems are many.
When using the Sonos Labs Plex service, the outgoing connection to Plex.tv for authorization often times out, or fails. That's right my friend, even from within your own network, Sonos forces you to dial out for all services - including Plex even though it just makes a round trip right back to your server again. You heard me right, LAN -> WAN -> LAN to play your local music...
If you get lucky and the service works, it most likely won't last long. That's my most common experience.
I get a good 5 or 6 songs played, then it just randomly errors out and stops playing. Even more commonly, the service authorizes fine, but then loading libraries takes up to a minute or longer, images don't load, songs fail to play, etc. (just check out the Plex Sonos forums for a taste of what to expect).
Additionally, the Sonos controller is so shitty to use, it takes all the fun out of browsing and playing your music and replaced it with frustration. Without Plex, the Sonos local music library section blows. But the Plex service blows equally hard...
After a year, the only way I can see that would solve all the problems is to fork out another $600 for a Sonos playbar and hook it up directly to my TV where Plex runs. Then I could use Plex (not the horrible garbage they farted onto the Sonos app store) to play my music properly.
Do what you're going to do, but I honestly listened to LESS music after trying to convert to Sonos because they have designed such an awful User Experience to go with such a smart and capable sound system. It's the worst commercial music app I've ever seen in the last ten years. Be warned. This is something Sonos hides really well from new buyers. There is a reason none of their advertising focuses on the actual control of the music - just the hardware. The hardware is great, but in order to use it you have to endure a kind of torture which is their app. Plex tried to remedy this, but gave up a couple weeks after introducing the service.
When the Plex service on Sonos actually decides it feels like working, it's a great experience. Just don't count on it being a reliable alternative to your existing setup (unless as I said, you forgo the Sonos+Plex service and just get the playbar and a few Play:1's and 3's for around your home)