I agree very much with what Eric said. To that I'll add this. The men who followed him during the Civil War loved him. Here's how one of them described him:
"Brave but not reckless; self-confident, yet modest; ambitious, but regulating his conduct at all times by a high sense of honor and duty; eager for laurels, but scorning to wear them unworthily; ready and willing to act, but regardful of human life; quick in emergencies, cool and self-possessed, his courage was of the highest moral type, his perceptions were intuitions. Showy like Murat, fiery like Farnsworth, yet calm and self-reliant like Sheridan, he was the most brilliant and successful cavalry officer of his time. Such a man had appeared upon the scene, and soon we learned to utter with pride the name of -- Custer." [James H. Kidd, Personal Reflections of a Cavalryman, p. 130]
"Stars of the first magnitude did not appear often in the galaxy of military heroes. Custer was one of the few. The popular idea of Custer is a misconception. He was not a reckless commander. He was not regardless of human life. No man could have been more careful of the comfort and lives of his men. His heart was tender as that of a woman. He was kind to his subordinates, tolerant of their weaknesses, always ready to help and encourage them. He was brave as a lion, fought as few men fought, but it was from no love of it. Fighting was his business; and he knew that by that means alone could peace be conquered. He was brave, alert, untiring, a hero in battle, relentless in the pursuit of a beaten enemy, stubborn and full of resources on the retreat. His tragic death at the Little Big Horn crowned his career with a tragic interest that will not wane while history or tradition endure. Hundreds of brave men shed tears when they heard of it--men who had served under and learned to love him in the trying times of civil war. I have always believed that some of the real facts of the battle of the Little Big Horn were unknown. Probably the true version of the massacre will remain a sealed book until the dead are called upon to give up their secrets, though there are those who profess to believe that one man at least is still living who knows the real story and that some day he will tell it. Certain it is that Custer never would have rushed deliberately on destruction. If, for any reason, he had desired to end his own life, and that is inconceivable, he would not have involved his friends and those whose lives had been entrusted to his care in the final and terrible catastrophe. He was not a reckless commander or one who would plunge into battle with his eyes shut. He was cautious and wary, accustomed to reconnoiter carefully and measure the strength of an enemy as accurately as possible before attacking. More than once the Michigan brigade was saved from disaster by Custer's caution. This may seem to many a novel--to some an erroneous estimate of Custer's characteristics as a military man. But it is a true one. It is an opinion formed by one who had good opportunity to judge of him correctly. In one sense only is it a prejudiced view. It is the judgment of a friend and a loyal one; it is not that of an enemy or a rival. As such it is appreciative and it is just." [Ibid., pp. 131-132]
Custer was an outstanding cavalry commander.
In the 1870s things were a bit different.
He allowed the 7th Cavalry to be fractured under his command. There were those he liked and to whom he gave preferential treatment. Others, like Frederick Benteen, resented it.
A good case can be made that responsibility for Custer's demise at the Little Big Horn can be placed on the cowardice of Marcus Reno and the outright hatred of him by Benteen.