As a distinguished contribution to children's literature, I can't argue. As a supplement to a unit on Medieval life, I don't know of any better. As a recommended title on the Help Readers Love Reading website, well...no.
Other books set in medieval times have won Newbery awards - Karen Cushman's books The Midwife's Apprentice and Catherine, Called Birdy come to mind - but Good Masters! Sweet Ladies! is unique in that it is a series of nineteen monologues and two dialogues rather than a story. Rather than focus on one point of view, Good Masters! Sweet Ladies! gives insight into medieval life from twenty-three different perspectives.
Although the forward states that these monologues can be read in any order, the published order is satisfying, with one leading nicely into the next. Isobel, the Lord's daughter, relates her distress at a gown soiled by a clod of dung thrown at her while walking through the village, then Barbary explains why she did it. Piers, the glassblower's apprentice, describes his anxiousness to learn the skill and please his master. Next, Mariot and Maud, the glassblower's daughters, tell how Piers has won their father's approval and that one may have to marry him. (After much internal debate, Mariot says she will. Maud says she'd rather have the plague or leprosy.)
While the numerous narrators accomplish the task the author intended - to give all her students an equal role in a medieval performance - and give it a uniqueness that makes it distinguished to the Newbery committee, it is this characteristic that keeps me from recommending it. Too many kids looking for escapability in a book aren't going to find it here. Teachers should use it when studying Medieval times. Assign it. Perform it. But I just don't see it being a widely recommended free read.