It's supposedly a rough draft of "To Kill A Mockingbird", evidenced by passages that were used in both novels. You may enjoy it and I definitely don't want to sway you or give the wrong impression.
So I've reread
To Kill a Mockingbird, and have read
Go Set A Watchman.
SPOILERS BELOW FOR PEOPLE WHO HAVEN'T READ THEM
I completely understand why people have such a hard time with the latter. And it took me a little bit of time to grasp why reading it pissed me off so badly.
After the end of the book, they expounded on the history. Apparently
Go Set A Watchman wasn't really a rough draft of
Mockingbird, nor was it really a sequel. It was a completely different book, written first, and never made it to being published. One of the publishers apparently asked Harper Lee to continue with the ideas, because there was a lot of merit there, and Harper Lee did so. In doing so, the characters grew in ways that made them incompatible with
Watchman.
So although
Watchman takes place with the same characters and takes place later chronologically, the characters had changed so much by the time that
Mockingbird was written that they weren't really necessarily the same people.
Reading
Watchman you feel betrayed that characters that you believed in were turned into something that you could never see them being after reading
Mockingbird. It feels like your heroes are being turned into villains, and you ask yourself "Why would Harper Lee do that?!"
But in reality, that's reversed. The characters who were villains in
Watchman were reformed BY writing
Mockingbird. But then
Watchman was published as-is, and was never made to reconcile with what those characters turned into.
So it's a sequel that was completely inconsistent with its predecessor, and thus people got angry.
Aside from that, I don't think it was nearly as well-written as
Mockingbird. The dialogue was much too glib, everything seemed a little too contrived.
But at least now I know what all the controversy was about.