Still worth finishing. S6 is not great compared to the other seasons, but its still Dexter. Season 7 is pretty good. S8 is not great either. I would say its a GoT level disaster BUT the ending is being redeemed by the ongoing continuation so that helps.
In subreddit r/Dexter
In a post titled:
After 5 seasons: should I bother watching the last 3? Is the new Dexter Series any good?