For all my bitching about ADWD, I rate the Theon plot in Book 5 as among the VERY best material in ASOIAF. Probably above anything in Book 1, actually.
I loved Theon in A Clash of Kings
a lot because it was refreshing having a POV character that was such a dick (He's WAY softer in the show!), but I never felt much sympathy for him as he made a long series of mistakes, never learning and was doomed from the start. In A Dance With Dragons
his plot completely broke my heart. "My name is Reek. It rhymes with freak." I also found Ramsay hilarious. I love it when he has a go at Theon for eating rats legally owned by Roose.
Really like the guy playing Ramsay in the show, even if he's nothing like the one I imagine or read.