I agree with Kanji. She may not be the best actor, nor is her character the most stable, but I like what Deb brings to the show and the dialogue that comes with it.
As a whole I throughly enjoyed season 5. It had a really, really long-ass build up, but it absolutely paid off. There was very little fat to this season. Nothing happened without a reason, not like previous seasons where there are retarded side-plots and unanswered murders. Like in seaons 4 when Dexter kills the photographer, but it's really the photog's assistant that he wanted. There was no resolve to that whatsoever. I feel like Chip Johannsen was given the reins of a tanking show (although season 4 was still great) and he had to make it work. In that regard, I think season 5 fit the bill perfectly. It gave Dexter closure with Rita. It allowed all the major characters to develop into more relatable personas. Plus it had it's fare share of awesome moments.
It may not have had the most thrills or surprises, but season 5 really never misstepped throughout its entire run, which is rare for a season of anything - let alone a Dexter season.