Well, it wasn't great, but I find it funny that people say "let Dexter go out on a good note, this is a great season", then "I'm never watching again" in the span of a week. It wasn't THAT bad.
I know a lot of you (I can't do it personally, my imagination is lame), sit around and think up how shows are going to end episodes or years in advance, and when it actually comes time for season finales, there's something awesome built up in your head that makes anything that airs seem lame.
Everybody wanted Deb to find out this season, so now that she hasn't, the season is the worst ever?
I do think that Season 5 was probably the weakest so far. This hasn't much to do with Deb not finding out about Dexter to do though. Not per se anyway.
As far as Deb/Dexter are concerned, I don't mind so much that Dexter managed to keep his secret... I do mind that it was done in a very lame "WTF??" kind of way. Which of course contributes to the overall lameness of this season.