It's just a massive DLC so I'll break it down into categories.
Gameplay: The gameplay isn't the selling point of Oblivion, nor is it the selling point of Knights of the Nine. It introduces some interesting new enemies, followers and spells, and has a very structured theme about it. Some of the fights are true Boss Battles (especially on harder difficulty) and are a lot of fun. But, as I said it isn't the focus of the DLC, and I feel that they could have touched on so many more enemies.
Lore: This is where the DLC really shines in my opinion, it brings some interesting new ideas and links into the Lore of the Imperial Religion and discusses some really massive characters including some awesome references and ideas (that I won't go too far into just to keep it spoiler-free) If you're a lore fan then this is definitely a nice DLC and the Lore is probably the most interesting part of the DLC
New Content: Compared to Shivering Isles or the Main game it adds very little. Some awesome new Armour and Weapon pieces are added, a couple of which have some really interesting quests and powers related to them. The dungeons feel a little different to Oblivion's other dungeons, but can be pretty copy/paste as about half of Oblivion's do. The quests are pretty much all fetch quests, but they're actually done in really interesting ways that really build towards the finale. I'd say the New Content most shines with the Quests really.
Basically it's a rather small but still excellent DLC that adds some new features that are awesome for Cleric/Paladin/Crusader characters but only passing for anyone else. The questline and Lore are pretty great but it could use some better level design.
Oh and if you've got Oblivion on PC, get Knights of the Nine Revelation, a mod that adds an epic story onto the end of this DLC which is far and beyond just about any quest mod I've played to date. I talk about it way too much 
Anyway, hope this helped a bit. Others might have differing opinions to me though.
It's just a massive DLC so I'll break it down into categories.
Gameplay: The gameplay isn't the selling point of Oblivion, nor is it the selling point of Knights of the Nine. It introduces some interesting new enemies, followers and spells, and has a very structured theme about it. Some of the fights are true Boss Battles (especially on harder difficulty) and are a lot of fun. But, as I said it isn't the focus of the DLC, and I feel that they could have touched on so many more enemies.
Lore: This is where the DLC really shines in my opinion, it brings some interesting new ideas and links into the Lore of the Imperial Religion and discusses some really massive characters including some awesome references and ideas (that I won't go too far into just to keep it spoiler-free) If you're a lore fan then this is definitely a nice DLC and the Lore is probably the most interesting part of the DLC
New Content: Compared to Shivering Isles or the Main game it adds very little. Some awesome new Armour and Weapon pieces are added, a couple of which have some really interesting quests and powers related to them. The dungeons feel a little different to Oblivion's other dungeons, but can be pretty copy/paste as about half of Oblivion's do. The quests are pretty much all fetch quests, but they're actually done in really interesting ways that really build towards the finale. I'd say the New Content most shines with the Quests really.
Basically it's a rather small but still excellent DLC that adds some new features that are awesome for Cleric/Paladin/Crusader characters but only passing for anyone else. The questline and Lore are pretty great but it could use some better level design.
Oh and if you've got Oblivion on PC, get Knights of the Nine Revelation, a mod that adds an epic story onto the end of this DLC which is far and beyond just about any quest mod I've played to date. I talk about it way too much
Anyway, hope this helped a bit. Others might have differing opinions to me though.
