I know little about W.T. Sherman, the man. What I know of him as a military commander is that he appeared to apply the concept of "total war" rather effectively. That is, he made little differentiation between military and civilian targets. They were all part of the same effort, the same machine that opposed him and his objective: To end the war.
Sherman wasn't content to fight a gentlemen's war where the combatants put away their arms when not on the field of battle. If the Union was going to defeat the Confederacy, it had to crush it, and not merely bruise it. The U.S. Navy was doing its part through the Anaconda Plan. Sherman and Grant were given the task of fulfilling the army's responsibility to see an end to the war as soon and as surely as possible.
Sherman is credited with having said, "The crueler it (war) is, the sooner it will be over."
He seemed to have a more realistic grasp of the nature of war, than the more naive and cavalier ideals held by so many that yearned to "see the elephant." To Sherman, seeing the elephant was akin to getting stomped by it. He saw war's ugly reality, and he frequently referred to it in the most unkind, but truest terms: "War is Hell," among them.
I think Sherman had the right mindset for war. Yes, his tactics were indeed brutal toward the Confederacy. He was also coldly cruel and brutal toward the American Indian tribes during the Indian Wars. But his strategies against his foes worked. Wars ended, the killing and the bloodshed stopped sooner rather than later. Later U.S. military commanders like Douglas MacArthur and George Patton possessed a similar regard for war. It was something needed to end something else. A means to an end. A bit Machiavellian, but effective nonetheless.
For this reason, I place W.T. Sherman highly among the USA's most effective and successful military commanders.
I'd rather have a Sherman in Iraq, Afghanistan or anywhere else our forces are called to fight, than a dozen General Lees if it meant ending the conflict sooner rather than later.