tl:dr version: The Sonos Amp is a cheaper way to go if you don't want a full-size amp/rack for your sound system.

long version:

I re-did my basement a few months ago and looked into going with a Sono's in-wall system.

If you are going in-wall and:
* want the 'Sonos' experience (app control) or have other Sonos equipment and want a unified home system.. the price isn't horrible for what you get, but there are other cheaper options.

* want the 'Sonos' sound... just know that the Sonos in-wall speakers are made by Sonance (not Sonos), and they are on the low-end side of the Sonance quality. IMHO for cheaper/better speakers you can get Monitor Audio in wall at a lower price (and still use the Sonos amp if you want.. or any amp). I wasn't impressed by the "Sonos In-Wall Speaker" sound. It required more speakers to get that 7.1 true surround experience.

At the end of the day... you are still going to have to get your HDMI and speaker cables to the Amp, and there are better options than Sonos.

I ended up not picking in-wall speakers... and went with the Sonos Arc, 2x Sonos Sub 3s, and 2x Era 300's. My goal was a simple yet great-sounding ATMOS setup. To get the same surround stage would have required more in-wall speakers than i wanted (ceiling and wall to get the full sound 'height/stage'). Its also easier and cheaper to upgrade when the next new thing comes out 🙂