The fourth was the last truly good season, after that it was pretty much all downhill (although I liked season seven more than five or six, because of Ray Stevenson). Season eight is just terrible beyond belief and for more reasons than just the finale.
Having said all that, you will likely find yourself watching the last four seasons and enjoying parts of them. They aren't 100% garbage, seasons five and seven have some good moments, six sort of just plods along slowly, is too predictable, and handles its themes quite poorly, Eight is a mess because the writers/producers don't appear to have been able to decide on what the focus of that season should be, so it lacks any sort of direction or natural flow as it jumps from one story to another. Plus as you've heard, the series finale is shit beyond belief.
It's a real shame the show declined the way it did, because those first four seasons (one, two, and four in particular) are amazing.