Having sunk hundreds of hours into Fallout 4, I figured it was time to buy the DLC pack. Quite a bit of that DLC just added stuff you could build. It's not something I would pay for, since mods do that much better. Here's my take on the rest.
The story line was short, but fun. I loved that they added some Silver Shroud lines at the end. The ability to upgrade robots was probably something above and beyond what mod authors could be expected to do, so that has value too. I could see myself spending $15 on this alone.
One of the big complaints about Fallout 4 was that you were railroaded into being a good guy, even if how you went about helping was questionable at times. Nuka World does add story and game play, but it seems like it is mostly there to let you be a bad guy. If you just start killing all the slavers when you arrive, you'll miss out on some of that game play. So, it's really not for me. I don't fault them for creating it. I will play it through, but would not buy it on its own.
Now this one is interesting. I loved how you just became a detective. Nick and Ellie just start referring to you as Nicks partner. You even have a couple of opportunities to introduce yourself as a detective. It feels organic in a way that nothing else in Fallout 4 has before. You have worked with him long enough that it makes perfect sense that you would just fall into place as this detective's partner.
I like the new area it added, and the completely different Scooby-Doo island feel it had. The fact that most of the island was unsafe for habitation actually made it feel more populated. One issue I had with Fallout 4 was how people treated you like you were a nobody long after you accomplished much more than any one person realistically could have. The things you do in Far Harbor feel like something one person could do, and the people seem to notice. If you're carrying a big gun they'll say, "With firepower like that, no wonder you did X, Y, Z..."
And I even liked the moral questions they gave you at the end. It was thought provoking in a way that the main story line of Fallout 4 wasn't. It even kind of rewrote that main plot by implying that you were an android all along. Which is super interesting, but too much to get into in this blog post. I would have totally paid $50 for this expansion on its own.