I thought the final episode was very good.
I thought it was a train-wreck, which I am sure was partially caused by the decision that Dexter must not have a happy ending for moral reasons. ( we can't have killers getting away with it )
Personally, I kind of wish that Dexter had ended with a sort of "Dark Knight Rises" ending, where Dexter has regained his humanity and is able to finally enjoy his life.