A few comparisons are in order. For example, Black Sails is highly reminiscent of the series Deadwood: both are dark, violent and based loosely on real people and real events. (In both series, some of the most unbelievable events are the true ones.) There are also similarities to the series Rome, as far as the strong characters, ongoing rivalries, and evocative period detail. But Black Sails out-does these predecessors in several ways.
First, the characters are, without exception, brilliant. Captain Flint, Jack Rackham, Anne Bonny, John Silver, Charles Vane - these personalities are truly memorable. Larger than life, yet always credible, always consistent. You probably won't recognize the actors, but they *will* make you forget Errol Flynn and Johnny Depp.
Second, the dialog is amazingly literate, almost Shakespearean. Without seeming artificial. Characters constantly engage in deep, intense discussions that are often more engrossing than the ship-board action.
Third, the intertwining tensions and plot-lines are maintained with absolute logic throughout all four seasons. We see winners, losers and casualties, all utterly believable within the bounds of the show. Power ebbs and flows among the various factions, and the conflicts always feel real.
Fourth, the ending is one of the most satisfying I've seen for any multi-season dramatic series. Everything comes together, as if you'd been watching a single, very long movie. What's more, despite the deaths, the violence, the many defeats that end this story, the series wraps up on an upbeat note and leaves the audience with a big grin.
As a bonus, the historical underpinnings are used cleverly. The show isn't constrained by the facts - it expands on them in a way that's both plausible and enthralling. Black Sails seamlessly blends history and myth, and even finds time to comment on the way they interact.
The staging is impressive, by any standard. The naval battles are believable, and depicted on a grand scale. The inevitable computer graphics look real enough to keep viewers immersed. The hot Caribbean sun permeates every frame of the series.
In short, Black Sails is one of the most thoroughly enjoyable dramatic series ever. It's great adventure, great drama, and by far the best screen representation of the great days of nautical piracy. This is one series you don't want to miss.