If you have a good-ish rig (as it's poorly optimized) and the patience for some annoying bugs, it's very good. The devs poured their everything into making London feel like a varied and dense place with its own identity separate from the Commonwealth, some underlying similarities notwithstanding (eg. no FEV means no supermutants but you still have loads of ghouls and ~~raiders~~ hooligans to kill), and it expands on visual story-telling that Beth Fallout games are actually good at. Aside from that, the quests are better (even if they sometimes fall into the vanilla shoot-n-loot formula), there are a lot of skill/perk checks in dialogue and I also quite like the companion roster and their unique quests. YMMV on some of the goofier factions (one of them are raiders who LARP as 18th century British sailors) but I personally like Fallout when it embraces some absurdity.

That being said, I wouldn't quite say it's Fallout 4's New Vegas like some have. It still has some fundamental flaws, some of which are actually worse than vanilla F4, like the main quest's handling of the faction conflict. Also the settlement system is still an unfinished mess and some of the original game bleeds through there.

There are planned updates which will include stuff for the main quest and even some "DLC" so who knows how much better it will get?