I have no experience with book Regis obviously, but I have to say that he was really, really well drawn here. Everything from his writing, to his character design, to his personality, to his actor's performance - man, I wish we could have had him hanging around in the main game. The two talks in the graveyard might might be up there with the best parts of the whole game.
A lot of the characters really popped in this expansion. I loved Anarietta, I loved Damien. Dettlaff was alright, but only alright. Syanna was pretty good, but only pretty good. I wish I'd seen a lot more of Orianna because she seems pretty coolly villainous, but I imagine you get more of her if you go after the hidden elder, which I did not. Over Hill and Dale was a great quest, though - the Big Bad Wolf, despite only having a couple of lines, really popped. He was hilarious.
I did get my happily ever after with Triss, which is great, but it was a little less affecting than I'd hoped it would be (I think the dialogue you get with her before the final battle was a little more poignant). And yeah, Kovir sounded nice, but I'd rather stay here. Maybe Yenn wins in that respect, because then you two get to settle down at Corvo Bianco.