Just saw Waitress at the ART. I'd give it a 6 out of 10.
Mueller gives a lovely, heart-filled performance - though, as was the case with ONCE, you have a grounded central character surrounded by stock, musical theater caricatures, for the most part.
Falls into the trap of most contemporary book musicals of being song heavy and not deserving to sing in each place a song is inserted. This is always something worked out by writers from tryouts to opening night. Also, falls into the the trap of many screen to stage adaptations where it is hardly a theatrical re-conception of the material. It feels (though I've never seen the movie) like a fairly straightforward, uninspired adaptation. Someone who has seen both may be able to confirm or refute this statement.
Most of the score is forgettable - but Mueller is given a couple Bareilles power ballads to shine - though on her penultimate number where she falls in love with her baby and gains the courage to leave her husband needs a dramatic arc (no fault of Mueller, its just the way the song is written).
I'd definitely recommend it to Bostonians who have a chance to see this pre-Broadway tryout - not so much to those traveling to NYC to see only one or two shows when they are in town.