I've really enjoyed Far Harbor. Yes, the location's theme (foggy forested wet land) is very similar to Point Lookout, but overall I feel the new story, enemies, weapons and characters are quite distinct.
And it definitely gives you more choices for completing the main story.
If you're pushing vanilla FO4 to its limit for content, then definitely give Far Harbor a go. Especially if you play on the new Survival Mode as Far Harbor seems like it would be a serious challenge.