The Treasure Island prequel angle is mostly just a gimmick, an excuse to use these characters. That would have been egregious had this show not done so well with the characters it used. Toby Stephens OWNS the role of Captain James Flint, creating one of the most compelling anti heroes of the 2010s, right up there with Walter White and Tony Soprano imo.
This is an action packed show with characters you can't help but root for, characters you fall in love with (lookin at you Calico Jack). You get invested in these characters lives. And when it's over you take a deep breath and know that you watched something truly rare: a good story from start to finish, one that starts and ends with purpose.
Also, I really appreciate how this show was basically a love letter to the Golden Age of Piracy. You see depictions of REAL pirates, and you get little tidbits here and there that only proper pirate nerds would know. Not to mention the painstaking attention to detail that the show prides itself on, with entire episodes focused on the specifics of pirate life that give you a real sense of how dangerous their lives really were.
*SPOILER ALERT* the inclusion of Mary Read right towards the end of the season finale served no purpose except to excite pirate nerds who would have read about her and wanted to see her portrayed.
All in all this show is a 10/10
P.S.
I will say though that the first season was a bit of a slow starter. It takes a while to really get the plot moving and the sex scenes are decidedly overdone. They kind of get annoying actually, but starting from the second half of the first season onwards the show gets intense rather than gratuitous and that momentum doesn't let up til the very end.