I don't like it. Granted I haven't seen him in anything other than F4, but I still don't like it.
If he hadn't been cast as Johnny Storm before it might be better, but the characters of Johnny and Steve Rogers are so different that it will be too distracting to take him seriously. For all of F4's problems (I thought so little of it I haven't even been able to bring myself to watch Rise of the Silver Surfer), he WAS Johnny Storm. maybe that signifies he's a great actor, but I don't think so.
It's a different swagger and a different character. Whether fair or not, he's Johnny Storm and therefore can't be Captain America.