I've pretty much finished the DLC; only a few side quests and locations left to explore. There's things I enjoyed and didn't.
What I liked:
The location and fog were well done, the new enemies a nice change up; I don't think there's anything pointing directly to King or Lovecraft, more mood than anything. They finally fleshed out The Children of the Atom to a point where I did not outright hate them. A friendly and intelligent super mutant that's tired of mindless fighting. There's plenty of unique armor to earn, that look like nothing else in the game, even a unique sledge weapon. Tons of new recipes to find. There were some quests that involved stat checks and a little problem solving [This game seriously needs to stop with the handholding]. There's quite a few chances to be an evil SOB, and a path where you pretty much can screw over the whole island.
What I didn't like:
And yet too many things from the vanilla game crept over: too many locations littered with boring super mutants, raiders, and feral ghouls. The fog didn't hit my PS4 too much, yet I did notice the occasional FPS drop or stutter. Some of the voice acting and writing could have been a touch better. For all the talk of how dangerous the island is, I hardly ever stumbled into a Fog Crawler or Hermit Crab. There's really nothing to do in the water, no dangers as far as I know. More settlements. Hurray. Speaking of settlements, I cannot believe they were stupid enough to let settlement attacks continue in the Commonwealth while I'm in Far Harbor. I'm halfway in a quest, and I have to go back and protect two settlements? And if I fail, I'm stuck having to figure out what random settlement items need repairs? This made me so livid. At the very least, settlement attacks should be placed on pause while gone from the area. And finally, the virtual reality puzzles. I did fine with most of them, even kind of enjoyed them, until the headache inducing final one. I had to take a break and return to it after my eyes started to hurt, yet managed to finish the stupid puzzle. And what was my reward? A set of gear I could have easily bought from The Children of the Atom. Lame.
Overall, I did enjoy this DLC, though it does have some major issues. Had they toned some things down [generic super mutant locations, the need for settlements, constant settlement attacks back in the Commonwealth], and beefed up others [Some better character writing, more involved quests and puzzles], this could have been great. The length seems to be around 20 or more hours, depending on how much exploring one feels like doing. The real issue is cost. This DLC is pretty much the size of the new location DLCs from Fallout 3 and NV. If I were to hazard a true price based on inflation, maybe 15 bucks? If I were to give it a grade, I guess something like 6.5 out of 10. Hardly the best Fallout DLC I've ever played. Honest Hearts, considered the weakest of the NV DLC, is still better than this.