If I consider Watchman on its own merits, I didn't find it nearly as shabby a book as I'd heard it to be. Granted, the characters are less appealing. Where Scout is consistently and wonderfully vulnerable and spunky, 26-year-old Jean Louise, coming home from NYC, vacillates between brittle and mouthy and self-absorbed. And Atticus is not the hero he is in Mockingbird; in this book, his willingness to stand up for Tom Robinson is attributed not from a sense that everyone, even a black man accused, deserves the best legal representation he can provide--that is, from a devotion to a humane ideal. It is the less personal devotion to the law. But these aren't real people, after all, and I didn't mind that so much.
What I found a bit surprising were some elements that felt incomplete, or inept. For example, I didn't like how Henry/Hank (why the switching between names?), Jean Louise's love interest seems to exist mostly to provide a thin "should I marry him or not" plot for Jean Louise; and as a character, he is flat, serving largely as a way to illuminate aspects of Jean Louise's character. (He often hints to the reader how we should interpret JL's speeches.)
The stakes in Watchman are not nearly what they are in Mockingbird, and this has to do with the plot. Certainly both books have to do with race and the south. But in Watchman, the plot is more about Jean Louise letting go of her father as a perfect model conscience, and figuring out that Henry isn't the one she should marry, than about race. In fact, much of what Jean Louise learns about race in the South is through a lecture that her uncle provides in the last 1/8th of the book. Thus, Watchman is a novel about an individual character more than a cultural concern, if that makes sense. A plot about a girl discovering her identity isn't as effective a way of introducing the problem of race as the trial of Tom Robinson (front and center in Mockingbird).
But I did like that Watchman ends in a way that doesn't put Atticus and Jean Louise (or anyone) neatly into the camp of Good or Bad. In some ways, it is a more "mature" ending, though perhaps not as satisfying as Mockingbird, in which Atticus transcends Maycombe's racism and Boo thwarts Bob Ewell's viciousness so completely. And if Jean Louise is a little too disparaging of Maycomb (in some wry scenes with the Maycomb ladies), that also feels "real" for a 26-year-old coming home from NYC. All in all, I'd give it 3.5 stars out of 5.