Its got grit, wit, and tits.
It may start off slow but it's not the action that brings you back its the mystery, plotting, and politics. As you watch you will slowly piece together elements from the past and why people do what they do in the present. The characters and their development are on a whole different level than anything you'll ever watch. Everyone seems to be out for themselves and those who aren't don't usually live long. Its a cruel world they live in but it's completely devoid of plot armor and supports realistic action like crazy. Pretty much if a character gets into a situation where he should die odds are he will die. Its like looking at a fantastical, medieval, Roman Empire.
The only warning I'll give you is don't get attached to any one character. Odds are tragedy is in store for them.