This expansion is better than the original game it is for.
In my previous review of Oblivion, I mentioned that graphics were the some of the best I had ever seen on a PC, back in 2007, and the physics were realistic but at times didn't perform very well. It was not the same experience as Morrowind, but was a good game in and of itself. The story was quick to beat, but there was much content to be explored, this game wasn't a waste of time.
The Shivering Isles, however, adds a more unique experience to Oblivion. It adds more of an Interactive story and purpose, rather than running around aimlessly, closing Oblivion gates to save the world.
There are interesting concepts of the Daedric Princes and their powers in the planes of Oblivion. Sheogorath is a prince, who created the world you enter. He is a character that you feel afraid of when in his presence. You know he could kill you at any moment, like any of the slaves in his twisted realm, yet he wants to keep you alive. He needs you for something.
The dream-like world you are in gets more and more odd as you progress. The characters you meet are just as odd. You feel as if you are in the Matrix, everything is an illusion, and you must watch your every move.
Gameplay and everything is the same as Oblivion, except for some exciting twists that happen in the game world around you. There are new armor sets, but I didn't even bother trying to collect them, they weren't the main focus for me. You won't even care about the new armor once you get involved with the story.
If you liked Oblivion, get this expansion, it's like playing a whole new game.