The first two seasons were the best. Then it lost its oomph and never got it back. It's a shame. Possum (or Locutus, I can't remember which) said the character had to develop but I disagree. A sociopath doesn't really develop. They are effectively disabled.
Dexter does need to progress to some sort of conclusion, either a bad ending (he's finally caught/commits suicide/goes out in a blaze of glory) or a happy one (he's cured or he becomes accepted by people he cares about - Deb, the kids - and continues his serial killing after the final credits roll).
I hate the idea of a bad ending for Dex but for him to be cured would be a betrayal of the premise. That leaves acceptance as the best ending for the show, basically some modified version of Dexter's rather grandiose fantasy of walking in full sunlight, from the final scenes of S1: