Graduating first in his class at West Point, fellow cadets nicknamed Lee "The Marble Man" because he just seemed too good to be true. As the story goes, he did not earn a single demerit in his four years there. So his caliber as a student of military science was almost as unquestioned as his discipline and brilliance on the battlefield.
He made few mistakes, but one of his weaknesses was his inability, at times, to listen to his junior commanders when they made suggestions about specific tactics and plans. General James Longstreet tried in vain to convince Lee not to attack at Gettysburg on the third day, arguing, correctly, that it was a suicide mission. But, as historian Shelby Foote characterized it, "Lee's blood was up", and you couldn't talk to him in that state.
Some historians say the southern loss at Gettysburg was the price the South paid for having Robert E. Lee as their commander. It is near blasphemy to suggest that among fans of the Confederacy today, and he is still a hero in the American South, but his decision-making in that three day fight was clearly flawed. There is little else to find fault with in his military tactics and strategies, and he was a very personable, charismatic and inspiring leader to his men and the country he fought for.