Part 1 is the first act, somewhat expanded. The additions are pretty seamless and add a bit to the story. Its been a long time since I saw the play, but there is only one part I was sure wasn't in the original

If you are into the play you would probably like it. I went with my wife and daughters and they loved it. I found it enjoyable enough. As someone who isn't super into the play, it was paced well, with some good comical elements.