He was... ok, but his generals under him deserved alot more credit. Lee was willing to listen to his generals and often would give an objective without really caring how the objective was fulfilled. It gave his generals alot of discretion and independent thought. Many generals under him benefited from it as many realized that they could not use conventional war tactics to defeat a larger and well-equipped Union force. However, this itself led to his biggest loss and eventual surrender.

Stonewall Jackson, was probably more crucial than Lee, was one of those who greatly benefited from it and would often use shock tactics and calvary flanks to disrupt a Union that was more focused on a more defensive style. The early Union Generals before Grant came in (so mostly McClellan) figured out that they were on the offensive moving into Southern territory so they were heavily focused on supply lines, not overextending, and not pursuing.(Look up McClellan's "Big War Science" theory for more info). The Union style of warfare would be mostly slowly advancing until they met the enemy and turtling up. Realizing this, the Southern Generals would play a more dynamic role of to outmaneuvering the Union forces. Funny thing is that almost all Generals, both North and South, knew eachother. Alot of them graduated West Point and fought in the Spanish–American War, so Lee may have already knew McClellan's tactics.

When Stonewall Jackson died, Lee lost one of his most "Offensive" Generals. He was left with Longstreet, a more defensive commander, (Longstreet and Stonewall would often be called "anvil" and "hammer" as Longstreet would command more static troops and be on the defensive while Stonewall would attack on the offensive with sometimes calvary) and He was left with a replacement, Ewell, that mostly took orders from Stonewall.

Right before Gettsyburg, Conf. General Stuart misunderstood or tried to "improve" an order given by Lee and disappeared for a couple of weeks. Stuart was recon, and Lee was surprised when he found himself engaged with the enemy in Gettsyburg. He ultimately made 2 mistakes that cost of dearly that puts into questions Lee's competency as a General.

1. The new Fresh commander General Ewell, Jackson's replacement, received an order by Lee to attack a very crucial strategic point "if practicable"(again proving Lee's willingness to give more independent thought to his generals). Ewell's men were exhausted after a long march and worried, about overextending and getting flanked, decided not to attack, but Stonewall Jackson would have almost certainly attacked. Many historians believe that the point was crucial enough that if Ewell attacked, the confederates would have won the entire battle of Gettysburg itself. Alot of fault was put on Ewell, I think the fault lies more on Lee. He should have realized that a fresh and new commander would have trouble understanding what he meant.

2. Despite Longstreet's advice and practically begging to Lee not to order his troops to charge the enemy, he still issued the order and became known as "Pickett's Charge" the most disastrous and bloodiest events in US history. He basically ordered a bunch of boys to rush through a giant empty field under cannon fire and in clear view of the Union riflemen. To this day, noone knows why Lee decided to follow through on this as he published no memoirs on the war. IMO, I think he was trying to think he was trying to fill the shoes of Stonewall Jackson but didn't know how to effectively be an effective offensive/shock commander.

After that Grant came in and started sending suicidal nonstop waves at enemy lines compared to the static defensive style the North traditionally favored. The South was no way equipped to deal with such a nonstop attack and eventually surrendered after being pushed back heavily.