Boost is lovely and polished and you can see that tons of work has gone into it.

Tell you what though: while I was waiting for Boost, damned if the Voyager (fka wefwef) PWA came out of nowhere with (I think) some of the nicest UX of any of the contenders, plus an insane release schedule because they can just push changes whenever. Voyager is honestly what has kept me here. (...he says, posting from Boost)