The writing has got so amateurish that the still-fantastic production values are no longer enough to save it. Season 8: this drop chasms down to depths you didn't think possible.
But at least the entertainment value is still very high, so it gets a pass. There are inexplicable ideas, inconsistent characterisation and soap-opera dramas. Season 7: you suddenly notice the writing and plotting has taken a sharp dive downwards in quality. GoT is worth watching for these seasons, definitely. Deep world-building, intelligent dialogue, epic set-pieces worthy of Hollywood, expert acting, detailed plotting, genuine surprises, great soundtrack used sparingly. What's the consensus? In this case, the consensus is accurate, and that is: Seasons 1-6: outstanding.
Let's say you're reading these reviews and haven't watched GoT yet, but now that it's finished are considering the marathon.