The praise for Jason Isaacs that I've read in other reviews is certainly justified but I would not go so far as some who declare that he looks and sounds just like the original. The accent is good but it's not exactly like the man himself (actually I think that Captain Scarlett and Fancy from Top Cat were both slightly better), also it rather comes and goes a bit. That, of course, could be deliberate given that one of the main themes is that the character Cary Grant was all an act. As for the visuals, from what I've seen Jason Isaacs looks nothing like Grant, but long hours in makeup have made him into a good imitation, most of the time. At other times he looks like Eddie Albert or Robert DeNiro.
One thing that keeps coming up about Grant is whether he was gay or not. That's not addressed here. It seems a bit unfair that his great chum Randolph Scott is sidelined to one small scene though.