My only experience with Frank Miller is with Daredevil. His work there is actually far more subtle and nuanced than his reputation led me to expect. Even though it's certainly darker than what came before, it built on the works of others in a way that made sense for the character. Had this been 1990, I'd have a decent amount of faith in his ability to pull of Captain America. After all, the good Captain appeared in his Born Again storyline and I thought he did a great job of capturing the spirit of the character (there was a bit of a "First Blood" storyline that was perhaps a bit right-wing in theme, but nothing too extreme).
Obviously, his recent work has cast doubt on all that, which is unfortunate. He is a great comic writer, but his flaws are certainly well-known as well.