Yeah, I've been warned that Dexter goes downhill and that S6 was simply terrible. I've only watched S1 and S2 so far but they were great and well worth the watch. I'm told S4 is the best. Each season of Dexter is a self-contained story. So I may just skip some of the later seasons. I'm kinda curious how they will end things this year though.

I found Sopranos even better from S4 onwards. You should definitely catch up sometime.