Forrest's fighting spirit makes for amusing reading - like the time he ran down and knifed the man who attempted to murder him, or the time he lifted a Union soldier onto his horse to use a human shield.
But as a human being he was repulsive to an extreme - slave-driver before the War, murderer of surrendered men at least once during the War, and Klansman after the War. In all fairness, there are claims that Forrest left the Klan when it became too extreme for his tastes, and he supposedly called for racial harmony late in his life.
I haven't done more than a cursory study of Forrest from a military perspective, but what little I have seen aligns with Viperlord's assessment. He probably falls into that category of historical figures who could be considered 'overrated' (whatever that means, exactly).
I can vaguely understand his appeal as a Confederate celebrity; compared to the immaculate, untouchable Lee, Forrest was rough and openly ruthless, yet probably much more approachable; being a self-made man, he is easier to admire and relate to. He seems to be the stereotypical racist, conservative, gun-toting white male American in his most gritty and spectacular form.
Definitely not one of the Civil War's more cuddly, politically correct heroes, probably overrated as a military man, and arguably a murderous racist bastard, but an interesting figure all the same.