No, he was really not great at all. There's been a misconception from the Lost Causer camp that has idealized some of these Confederate generals and overrate them, when they did nothing to earn it. What did he do to earn a place in the pantheon of great generals? Nothing. IMO, is should be placed in the pantheon of underperformers because he did absolutely nothing to stop the Union army from tightening up the Mississippi. The reason why I consider him an underperformer is because he had the advantages to emasculate the Union Army but never remotely did. He had the classic triad for guerilla warfare: an analogous net of sympathizers, supporters and actives. Intelligence is critical to irregular warfare. Wherever the Yankees went, there were rebel eyes watching them and then broadcasting over a network of neighbors and friends, a web connection over which news traveled fast.
From the perspective of the twenty-first century, it's easy to see that such insurgencies were and remain capable of paralyzing whole armies, turning them into targets emasculating them strategically, and that's because modern insurgents just did that and were carpet bombed into almost oblivion. Yet the main reason Sherman never realized the potential and danger was there was because that it never materialized. For all their tactical success, Forrest and his raiders never succeeded in bogging down the Northern invaders, from piercing the heart of the Confederacy. Forrest and his insurgents underperformed. And from an understanding of irregular warfare, this is puzzling. Therefore, what did Forrest do to be considered great? Nothing. He didn't even stop resources from making it to West Tennessee, and the west, at best he diverted resources.