Here's the problem, though... Anyone who knows anything about Grant is going to have a significant amount of pre-formed opinion as to how he should be portrayed (actors playing Lee have a similar challenge, though I think Lee has had more screen time than Grant), and he's not as easy to caricature as (say) Lincoln or Stonewall Jackson (or to cite a recent non-Civil War example, Churchill).
I mean caricature in the sense of exaggerating widely-known aspects of the look or the personality. Let's face it, have any tall, thin man grow a beard and put on a stovepipe hat, and he'll be almost automatically accepted as a credible Lincoln before he even opens his mouth... Apart from the issue of the drinking, there's nearly nothing that's widely known about Grant in the popular mind, apart from being on the fifty and showing up in the Wild Wild West...
So, basically, I don't envy the actor selected to play Grant; he's going to be roundly criticized no matter what he does.
ETA Just re-read this and my point sort of got lost in my meanderings. What I primarily mean is that, in the popular mind, Grant isn't as much an icon as other historical figures. So people with little advance knowledge won't really know what to expect, whereas those with pre-existing knowledge will be more ready to jump on every aspect of the portrayal... does that make more sense?