Ule’s already discussed this briefly in the DLNA topic.
First, though, it’s important to know that Sonos’ big differentiator from the beginning has been its synchronization of play across multiple speakers. You can either play the same thing on multiple Sonos units at once, or have music follow you around the house.
Echo very notably lacks this. People who have more than one often mention this lack.
So Echo as a bluetooth speaker, yes. But not so much a “Sonos alternative.” Of course that’s true of most of the other bluetooth speakers also. 