TL;DR
Buy it if you prefer the individual levels to the overal world or the bosses. If you prefer the gameworld, buy Dark Souls 1. If you prefer boss fights, buy Dark Souls 3.

Compared to Dark Souls 1, this game is much more consistent in its quality, with lower highs but higher lows. There's nothing in DS2 that's awful on the level of Lost Izalith, but there's also nothing nearly as good as the Painted World outside of the DLC. However, with the DLC included for free in the SotFS edition, the game is elevated to much greater heights. Sinh, Fume Knight, and Ivory King are among the best bosses in all of Dark Souls, better than almost every fight from DS1 and a good chunk of the DS3 boss pool as well. They also contextualize the story of the game in a way that makes the game far more personal -- it's much easier to be invested in the world of DS2 compared to DS1 and 3 where the stories feel like a backdrop rather than motivation.

COMBAT
Combat is Dark Souls combat, I don't feel I need to explain much more than that. It lends itself more to larger groups of enemies, and magic is greatly improved, but it's still Dark Souls, and it's just as good as any other Dark Souls game -- faster than DS1 as your attack options are better, but slower than DS3 because multiple enemy encounters require patience and picking a good time to attack -- the Dual Pursuers in NG+ play more like Ornstein and Smough than as a generic combat encounter. PvP is also great, but my experience is too limited to comment on it in depth here.

PRESENTATION
The game is, unfortunately, ugly -- much less creative enemy designs than DS1 and 3, with most of the new enemies being fairly standard humanoid enemies or bland looking 'fantasy monster' -- Ogres, those weird mushroom things, and the long-armed green enemies in No Man's Wharf are all forgettable enemy designs, a far cry from the unique appearance of the basilisks, the unnatural bodies of the mimics, and the horrifying bug/human hybrids in the depths of Blighttown. Boss design suffers from a similar lack of quality -- there are no fights as visually striking as Sif, Gaping Dragon, or Gravelord Nito. The most creative boss design is the Executioner's Chariot, an optional boss that culminates in a fight against a pretty generic horse enemy with dodgy hitboxes.

LEVELS & BOSSES
Speaking of bosses, outside of the DLC the boss design in this game is downright bad. I can honestly say that there wasn't a single fight that impressed me in the entire base game. In stark contrast, the levels between these boss fights are fantastic, and with a couple exceptions they were all well designed. I didn't like Heide's Tower or Earthern Peak, and I still can't tell if Shrine of Amana is a great zone or an awful one, but every other zone felt satisfying to fight through and explore, with plenty of secrets to discover and interesting enemy set ups that forced me to adapt to a very steady difficulty curve throughout each 'path' -- the first zones, like Forest of Fallen Giants or Shaded Ruins, were generally pretty simple and straight forward, with some kind of mechanic the zone wanted to teach you, be it curse pots, poison build up, combat against large enemies, combat against many smaller enemies, etc., but each level path would iterate on these ideas, often culminating in a final stretch to the boss that is a grueling test in all of the lessons you've learned -- the cave before Freja, the flooded hallway before Sinner -- or in an optional but very difficult boss with an even more challenging run-up -- Smelter Demon, Belfry Gargoyles if you're online -- that makes the rest of the zone easier to tackle in some fashion.

WORLD
The most conflicting part of this game is whether or not I hate the gameworld or adore it. Dark Souls 1 had an amazingly interconnected world throughout the first half of the game, but threw this concept out the door in the second half besides a few cool moments, like seeing Izalith through a hole in the wall in Tomb of Giants. Dark Souls 3 didn't have many interconnected paths between zones, but instead focused on feeling more like a journey than a quest, moving through the world in a linear fashion with a tightly controlled difficulty curve. Dark Souls 2 has multiple paths that you can tackle in almost any order, allowing you to tackle the game the way you choose to -- and not in the "well, technically" fashion of Dark Souls 1, but by having a more consistently challenging game that retains its difficulty regardless of what path you go down. However, these paths do the infuriating and yet somewhat fascinating thing of refusing to make any logical sense. You walk through tunnels that teleport you from daylight in a forest to rainy nighttime on a mountain, you climb elevators that lead from the top of a tower upwards to the base of a volcano that doesn't even exist, and your tutorial zone is called Things Betwixt, as though its lodged between worlds in some way, and the only exit is through a crack in the barrier between the world of Drangleic and the world of Things Betwixt. If it wasn't a concept that was explored further in Dark Souls 3, culminating in the complete devolution of time and space in The Ringed City, it would be easy to call this lazy design, but its clear in retrospect that this game is very intentionally building an atmosphere of unreality that is utterly fascinating to look closely at once you start to pay attention to it and look at how many ways the game's story and world ties into this idea.

DLCS

Sunken King plays like a Zelda level, with a bow and arrow being necessary to explore the level. Most of the bonfires are hidden and well spaced, and I could see somebody getting through the whole DLC only ever finding two of them. Enemies are challenging to fight, and the ghostly suits of armour are a very interesting challenge that I won't spoil, even if I struggle to imagine being able to defeat them all on just one life. Sinh is the best endurance fight in the series, and possibly the best dragon fight in the series -- I'm still torn on whether or not Midir is the ultimate challenge or just obnoxious -- with his movements making him difficult to follow across the room and his damage being difficult to avoid. Taking him down feels like a triumph simply because the odds are so stacked against you, with the durability damaging armour plating, the toxic building flame he spits at you, and the massive hitboxes of many of his attacks.

Old Iron King has the best level design in any Dark Souls game ever, with an absolutely perfect understanding of how to pace the DLC. The DLC only has one mandatory boss fight, and he is an incredible challenge, and certainly the best humanoid boss pre-Dark Souls 3, where the humanoid boss bar was raised so high that nothing could ever reach it. When you activate the elevators that span the tower, what was previously an incredibly difficult to navigate maze becomes much more accessible, and dozens of secrets are opened to you. Incredibly satisfying level design elevates this DLC to the top of the pack.

Ivory King is the worst of the DLC, but has the perfect fight to end your DLC experience with, as Ivory King is the most gorgeously presented boss in the entire series. However, exploring the level twice after melting the ice feels tedious, and the Frigid Wastes are a war crime. Although great, I spent more time being lost than making progress in this DLC.

FINAL THOUGHTS
Overall, this game is a solid 8/10 for the base game, weighed down by poor boss encounters and bland presentation but carried by a top-notch combat and progression system, but a 9/10 once DLC is included, with the drought of good bosses being augmented with several fights ranging from great (Aava, Elana) to fantastic (Fume Knight, Sinh, Ivory King), the story being heavily improved, and the already great level designed being honed to perfection.