For many years, my sole opinion of Custer was based on how he met his end. Over a long period of time, I have gained a grudging respect for him. I no longer dismiss him as a lightweight as I once did. The truth is that he is a very complex and deeply interesting character.
He graduated last in his West Point class by choice. He knew he wouldn't graduate first, so he figured, why not finish last? On those occasions where he came dangerously close to expulsion, he straightened up, flew right, did extra guard duty, and worked off demerits. Somewhere along the line, he contracted a social disease at Benny Haven's that rendered him sterile (which is why he and Libby never had children). He had briefly been a school teacher before attending West Point. He definitely had a serious side. He was a teetotaler, and he did not curse or swear.
His biggest problem, I think, was the fact that he jumped up the ranks without paying his dues. By that, I mean that he went directly from being a very junior staff officer to a brigadier general. Other than one raiding expedition, he had never commanded anything. He had no knowledge or understanding of regimental politics, because he never served in one before becoming a general. He did not know how to play army politics, and it nearly cost him his career after the war when he testified against Grant's Secretary of War, Belknap, thereby pissing off Grant and nearly costing Custer his entire career. There's a reason why 11 years after his promotion to lieutenant colonel in the Army, he still had the same rank.
Because he did not know how to play army politics, it made him very unpopular with most of the officers of the 7th Cavalry after the war. Other than those who were in his inner circle, they never forgave him for leaving the very popular Maj. Joel Elliott's body behind at the 1868 Battle of the Washita, and it is difficult to describe the depth of Fred Benteen's loathing of Custer for all of the reasons stated herein. Again, there is a reason why he was still an LTC 11 years after the end of the Civil War.
Also, because of the circumstances of his promotion, he had no experience with, nor any particular talent for, the traditional roles of cavalry: scouting, screening and reconnaissance. But he could fight. Make no mistake about that: he was a fighter, and in Sheridan's world, that was the primary prerequisite. At the same time, Custer's impetuosity nearly cost him his brigade on the first day (June 11, 1864) at Trevilian Station when he blindly charged into a Confederate wagon park and soon found his entire brigade completely encircled and having to fight its way out. The legendary Custer's luck was with him that day; he was struck by a spent bullet that merely bruised him, and he managed to hang on until his rival Wesley Merritt cut his way through to Custer and relieved his command of the pressure. I call it "Custer's First Last Stand", and I think it's an accurate parallel.
Part of Charles Francis Adams' description of Kilpatrick equally applies to Custer: "a brave, injudicious boy, much given to huffing, and will surely come to grief." He was full of bravado, had an immense ego, and was prone to acting without proper caution. He was a brave and inspirational leader, and the men in the ranks loved him during the Civil War--they would have followed him anywhere if he led them there. There is definitely something to be said for leading from the front, and there's no denying that Custer led from the front. Sadly, the exploits of a 23 year old boy don't translate well to a 36-year-old veteran of 15 years, which is what Custer was at the time of the Little Big Horn.
In the end, it is exceedingly difficult to argue with his record in the Civil War. His rise and accomplishments are nearly without comparison. But that's the thing about luck--when it's your primary asset, eventually it runs out, and you're left to fend for yourself. And in the case of Custer and nearly 300 men of the 7th Cavalry, the luck finally ran out on June 25, 1876.