Anything Oblivion is always fun. Knights of the Nine is fun, by quick and pretty easy, for an experienced player.
it's fun -- very easy for my level 26 Battlemage (even combo of magic and melee). I got to add a couple levels. The quests are very straightforward. You won't be searching around a lot for secret places. You go to nine spots, nine quests, get the armor and weapons, kill the bad guy and you are done.
The weapons and armor and nice additions. I have the great Duskfang and Dawnfang sword from Shivering Isles, so I used that much of the time. But I used the Crusader sword for the endgame.
Shivering Isles is a LOT better. Much more character development and good plot twists. But I had fun with this one.