I'm not sure whether I prefer Iron King over Sunken King, but so far I'm leaning towards Iron King. They have many of the same strengths/downsides, but Iron King I find more aesthetically pleasing and the more tangible lore connection is a definite bonus.
What needs to be said is that - like the first DLC - this is an area driven DLC. That is, its greatest moments come from its environment, its (regular) enemies, its traps and puzzles. That aspect is very similar to the underground pyramid platform festival that was Sunken King. It is NOT boss driven, and could disappoint in that regard. I think of it like this: back when I first played Artorias of the Abyss for the first Dark Souls, I enjoyed it a lot, but in retrospect it was entirely for the bosses and the new items/equipment it brought to the game. While some sound aesthetics were neat, it consisted of an area that was re-used with minor changes set at day, an ugly, blurry series of buildings (which had some cool areas but not anything as nice as what was in the base game) and finally a third rate Tomb of the Giants. The DLC for Dark Souls WAS boss based, and that's the main difference here. There's enjoyment to be found, but in different ways.
Iron King is larger than Sunken King overall (more around the size of Artorias of the Abyss) and has plenty of From's intricate detail sprinkled all over it, visually and audibly. The snowy towers make what would otherwise not be so great more interesting by having connections via tiny chains, and each one leads to an area that is a complete nightmare (curses, ambushes, etc.)
Without spoiling any lore details, the interior areas look appropriately similar to those of Iron Keep, albeit more metallic. Things move, rotate, and power the giant fortress. Inside you'll find plenty of items and enemies. It's the things that make connections to what was not fully explained in Dark Souls II that have this DLC make up for its shortcomings (for the most part). There's also some nice gear, including one particular sword that people who missed it from Dark Souls will probably enjoy. The DLC also has a cave area similar to Cave of the Dead from Sunken King. At first I was disappointed to see this - while visually different, it is almost the same concept - however, going through it a couple times, I no longer felt that way. It seems as though it was an attempt to make up for the weakest area in Sunken King, and it does make up for it. It's the kind of place so danger ridden you can't even stop to look at some of the new enemies trying to take you down. The boss of that area was where the first "meh" from the DLC came in. It's a boss you've fought before...sort of just a bit different. In retrospect, I don't hate it, because this DLC ties to an area from the base game heavily and as such the presence of this boss (in a different sort of state) is appropriate. However, they could have done more to change it up.
Beyond the cave area (which like the last one is optional, I believe), the rest of the DLC is good. The bosses may leave you wanting more, but it excels with its environment. There's a sense of scale to it that you don't get a whole lot in Souls, and though you can't traverse all that you can see by any means, there's still enough there.
Ultimately, I give this DLC about an 8/10 and a recommendation. It's not groundbreaking in any way, shape or form, but it is an area that is extremely welcome in the Dark Souls II world. Just remember what I said about the area-driven/boss-driven thing. If you want to explore a snowy tower with loads of enemies, pathways to other sections that extend over great heights, remnants of the past and some interesting new puzzles (without really spoiling anything: boom), you will like this DLC. However, if you're only wanting to get something to fight new bosses and don't really care about the setting, you might skip this one and wait for Ivory King.