At one time McCellan was a very popular target of the Lost Cause school of Civil War history, which would contrast his cautious and slow strategy with Lee's dashing tactics. With the demise of the Lost Cause , there's been something of a shift in that, and historians have been more able to look critically at Lee and some, like Ethan Rafuse, have tried to rehabilitate McClellan somewhat. Both Rafuse and James McPherson have noted that the general strategy of the Union generals early in the war to occupy and secure Southern territory was at odds with Lincoln's goal of destroying the Confederate armies. In McClellan's case, this was consonant with his political beliefs: he was not an abolitionist or a Republican ( his family were Whigs). He thought that it would be possible to negotiate an end to the war, and avoiding unnecessary bloodshed therefore made sense. We can now say that he was wrong and Lincoln was right, but it is fair to say that, at the time, quite a lot of people shared McClellan's point of view. Certainly he was very popular with the army when Lincoln removed him.

McClellan's diary and letters to his wife have show him as egotistical and vain, and contemptuous of Lincoln. He was- but Rafuse has pointed out that he had malaria and was often ill- notably on the day that Lincoln tried to meet with him and couldn't get an appointment, a famous story that has often been told. He disliked Lincoln, but on that day he likely had a decent excuse to not take a meeting with him.

But, just to focus on his redeeming qualities: there is no doubt but that he created a pretty good army, that was drilled, supplied with guns and tents and food and equipment, had ambulances to pick up the wounded, surgeons to treat them, and had pretty good morale. That army was later used to great effect by Grant and Sherman. By contrast, Lee paid little attention to medical care for his men and little attention to feeding them. He would take them north in 1862 with no supplies, expecting them to beg from any house they passed or to raid a cornfield for green corn. After the Battle of Antietam, the residents of Shepherdstown found themselves running the town as a field hospital, with little or no help from the few Confederate army surgeons who finally appeared. Logistics may not be as dashing a subject as tactics, but many will say they are at least if not more important, and you can certainly say that McClellan was better at them than Lee.

McClellan was an overall poor field commander, that is not really in doubt. He constantly believed extremely poor intelligence (often believing himself outnumbered when holding immense numerical advantage), he was overcautious, and he was inflexible. On a personal level, he was an egotist who had political aspirations, which didn’t help with the military side of things.

All that said, he was also a talented organizer. He took a disorganized and defeated army consisting mostly of short-term volunteers and molded it into a vast and effective fighting force. As another poster mentioned, he kept the army amply supplied with food, ammunition, equipment, artillery, shoes, horses, etc, and built an effective (for the time) medical service.

As a general, he did have some minor successes. In western Virginia, he commanded at the Battle of Rich Mountain, but it must be pointed out that he had almost a 4-1 advantage in numbers. His decision during the Seven Days to withdraw to the James River rather than to the east was strategically important because it foiled Lee’s plans, but it’s debatable how intentionally this was done. The Army of the Potomac won the Battle of Malvern Hill while ostensibly under McClellan’s command, but in reality he was miles away on a gunboat. At Antietam, he won a strategic victory of great importance, but he did so almost despite his command rather than because of it. Regardless, he deserves some credit for turning Lee out of Maryland, regardless of how it was accomplished.

I am not as familiar with his post-war political career, but he did serve a term as governor of New Jersey as well as several positions within NY state and city, so one could argue that he had success in those areas.

/u/petite-acorn has previously written about the historiography of McClellan's reputation alongside /u/67thtigers.

Petite Acorn also explained why McClellan's men were so loyal.

/u/louwilliam has previously discussed Lincoln's choice of generals more broadly including why Lincoln liked McClellan.