That depends on how much you care for Fallout lore. If you liked the classics and FNV, you might find it disappointing. If you liked FO3, you'll love FO4.
The gameplay is certainly better and very fun. Settlement building is interesting, but ultimately doesn't tie very well with the rest of the game. Gunplay is good, but melee is mediocre (worse than Skyrim, which is surprising).
Graphically it's an improvement in every front. Shading, lighting and especially design - even if it's more different from past games, the world's very colorful and engaging (what you couldn't say about FO3/FNV without ENB).
Plot-wise... it's a mess. A hot garbage mess. Bethesda basically rehashed FO3 plot and made it... not worse, but certainly more dumb. Same goes for most side-quests which serve no purpose other than "go-shoot-dudes". (f.e. - FNV made almost every quest in some way tie into the world and conflict between NCR and Legion). There's only one-and-a-half town, which is a big minus, as most of the map can seem boring.
As for DLCs - Automatron is lame, but adds robot construction. Nuka-cola is cool if you like amusement parks and raider gangs with some personality. I haven't played Far Harbor.
So, in final words, if you have spare cash and it's on sale with a good deal - play it. It still scratches that itch of open-world exploration that Skyrim and most Bethesda's games offer. But don't expect to find anything smart in it.
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