Mod adds 1-1.5 to Skyrim's game score, needs much more publicity then it now gets
Now, unlike all the people who slag Skyrim off for omg-so-horrible-bad-and-unimmersive writing, I find the writing in it to be... adequate. It had some inspired moments, some flat ones. Some situations were interesting, others were too vague.
Then I delved into 3dnpc. And with it included, Skyrim actually becomes a game where character writing is good, rather then just "adequate".
The standard of writing is high, and adds to the atmosphere of places where characters live even if they offer no quests. To give you an example, this mod adds a female Dunmer in Windhelm who's a cousin of one an existing character (Belyn Hlaalu). Her story about an unrequited love with a Nord adds some tragism to the Nordic-Dunmer relationship of the city, and her thoughts of convincing Ulfric to improve the Grey Quarter though diplomacy and negotiations expand on both her own character, the character of other Dunmer inhabitants of the city, and the nature of House Hlaalu.
The added quests not only expand on existing locations (thought that College of Winterhold should be more prominent in the game? 3DNPC has an average-length quest line which starts in the College, and a minor quest also is involved with it! Thought that Windhelm needs a major quest to gain more flavour? There is one!), they also have emotional involvement. The College of Winterhold quest starts with a request by a mage to find her missing Khajiit assistant. Said companion's dialogue about how the mage recognized his magical talent while watching him perform magical tricks as a beggar, and how under her tutelage, he was able to develop his talent, his guilt about a kind-hearted woman who helped him as a beggar and whom he later snubbed make for a very immersive listening.
The only major flaw of this mod is that as a result of its development cycle, some quests and questlines have rather arcane unintuitive pre-requisites (e. g. in order to start a questline about an ancient spiritual order and a sympathetic character from Dawnstar's missing-from-childhood friend, you have to kill a spider in Markarth for a character from the original game who, story-wise, has nothing to do with the questline. Also, ask innkeepers about "interesting news or rumours" often).
Overall, this mod, by its characters, enriches the game world in general. Highly recommended.