I think it was outstanding. While it is in fact quite lengthy, the pace at which it needs to move compresses it considerably, and its three-hour run time could easily be five or six.
I wish more time could have been dedicated to Oppenheimer's earlier life, as well as more interaction with some of the greatest scientists who ever lived. But, there simply isn't time.
Know going in, while the movie does involve the creation of the device, a great deal of time is spent with Oppenheimer and Lewis Strauss being at odds, and the emerging strife between them post-WW2.