I wasn't a fan of the Cat Stark character in either the book or the show, though I think Michelle Fairly did a fantastic job portraying her. HOWEVER, I thought the Robb Stark plotline through Cat's eyes in Storm of Swords unfolded beautifully, in that everything turned from great to dire, literally overnight... they way he could "win every battle but lose the war in bed" was one of the most intriguing storylines of the series... and of course, every SoIaF fan knows how that storyline ended...