As a six hour movie (yeah right), how bad the whole saga is depends solely on ROTJ. Even if you see that film as flawless, the whole saga, as represented as a two hour movie, would only be 50 percent good. At worst, it would be a bad movie with 40 good minutes in it.
And don't get me started on the sequel trilogy; by 2025 we could have a saga that is over 80 percent bad!