Well, from my armchair (I'm not literally sitting in it right now, but I do have one-- a 100-something-year-old Morris chair that was my great-grandpa's), here are some of my thoughts assessing Grant's performance as a general in the Civil War.
Pro: Grant seems to have known, instinctively or by experience, of the importance of combined arms. As usual, I'm thinking about this primarily in naval terms, but on the Western rivers his cooperation with the Western Flotilla/Mississippi Squadron was almost uniformly smooth and effective (and believe me, Porter was not the easiest person to get along with), and he continued to exercise that ability when transferred East. He appears to have known that the Union navy was a competitive advantage against the Confederacy, and operated with it effectively with few bumps or disagreements (a notable exception being his insistence upon obstructing the James River above City Point, over Rear Admiral Lee's objections).
Pro: From my reading of Grabau and others, it seems very difficult to escape the conclusion that Grant's force had an effective intelligence-gathering organization in his operations in the West. The situation may have been different in Virginia, where he was mostly reliant on someone else's (Meade's) organization and other conditions were different.
Pro: From every source I can recall, it appears that Grant had a cool head and a large amount of personal courage. Though not glorified by his troops in the way that a McClellan or Lee was, they appear to have been generally fond of him and confident in him. I haven't done as much reading on this as I intend to, but from outward appearances it looks to me like Grant knew the value of good staff work and had an effective staff around him.
Con: Grant doesn't always seem to have been able to accurately estimate the value of his subordinates. Some he thought the world of, for little apparent reason; and some he did not treat fairly. I could be off base on both of these examples, but it seems to me that McPherson and Rosecrans are appropriate examples. I still am not clear on what Grant saw in McPherson as a commander, as his performance seems to have been quite mediocre; whereas Rosecrans, someone with clear abilities and a good record, ran afoul of Grant and was treated poorly, both at the time and in after-the-fact recollections.
Con: Though some of the problems in the Overland campaign actually really belong to Meade and his staff, Grant was in top command and could have directed things differently. It seems to me that a lot of the frontal-assault business usually blamed on Grant really stemmed from the way the Army of the Potomac operated, and that he probably should have concentrated a lot more on the maneuver operations that he demonstrated aptitude with in the West. (Important caveat: in the West, he was not up against a counterpuncher of the undoubted ability of Robert E. Lee, and this undoubtedly had a major effect.)
So... neither an unstained hero nor a feckless bumbler.