It's honestly a bit on the more expensive side, but the quests are fine and the whole experience doesn't just swamp you with meaningless BS for 20 hours, so it's still got like a typical FPS campaign's worth of time.
There's 2 main quests: help Far Harbor and help the Synths. The first is mostly optional, the 2nd isn't, but doing the first will have an impact on the 2nd. There's multiple endings, several factions, etc. All pretty good.
And you can even eliminate at least 1 of the factions, which is good. You can't kill certain main characters, though, not even after completing their quests, which is bad. Guess, we will "never know" whether Kazumi really is a Synth or not. Although, it's very likely she isn't. People keep saying you can kill her, but I couldn't. She just goes into a downed state. Something which I really hate about FO4.
The true star of this xpac is the island, though. It's moody, it's atmospheric, mist keeps wafting in from the sea, monsters lurk in the darkness, super mutants, crazed zealots, mutated wildlife and bandits are everywhere, and you can actually SEE the map, instead of just 10,000 location markers right next to one another. It's just got a way better layout with actualy interesting locations to explore, instead of copy-pasted interiors. It's probably the best FO4 has to offer, and it still suffers from you just being a "yes/no" kinda gal or guy, but at least there's some actual nuance in the choices you can make here, instead of most of the stuff being binary.
Fallout has always suffered to an extent from being "just" the wasteland. It's big, too big, with little variation in scenery. Far Harbor does an excellent job at remedying that.