I have never been a big Stannis fan, outside of the occasional dry quip he makes, so I'm fine with his characterization. I think it will gradually get closer to the book as we get closer to him saving the Wall. He's just as gruff and quasi-dull as I remember him being in the book though, so far (been a while though).
Although admittedly, I'm fine with most of the characterizations in the show, and the combination of characters. The show would be 30 seasons long if they tried to do everything, and show-watchers already have a tough enough time keeping shit straight.