As others have noted, Jason Robards' performance as Brutus has real problems, particularly at the start. While playing the character as vague and distracted is certainly justified by the text, Robards often appears just this side of comatose. James Mason was far better in the older film version.
The biggest and best surprise is Heston's strength as Antony. He believably conveys an interesting mixture of loyalty, craftiness, and ambition. And his work in the funeral oration is well-paced and well-directed. Overall, this version has a lot going for it -- it's among the better Shakespeare adaptations that I know of.