I stopped watching after season 4 because I hated Ghost Harry and because they had to make Dexter incompetent for the whole season to justify the entire plot.
I have heard what's gone on in future seasons. What I've heard makes no sense with established character traits. They've been in 'Reaching' territory since season 3 or so. The show lost its visual distinctiveness and creativeness after season 1. They never had anything nearly as cool as the hand suspended in ice or the blood-soaked room after season 1, and also all the characters moved away from their original personalities toward the center.
Still a great season 1 and good season 2, though.