The Shivering Isles is a unique and highly enjoyable addition to an already spectacular role-playing game.
The Shivering Isles adds a whole new world to the game (called, not surprisingly, The Shivering Isles) that can be accessed from an awesome-looking portal in Cyrodill. When you enter the Shivering Isles, you stumble upon a rich, vibrant world filled with new allies, enemies, daedric gods, armor, weapons, and all sorts of delectable madness. Expect plenty of the latter, as The Shivering Isles are ruled by Sheogorath, the Daedric Prince of Madness (quite an interesting character). When you meet him, you learn that the Daedric Prince of Order has started something called the Greymarch, which threatens to destroy the beautiful madness of the Shivering Isles, replacing it with hideous order and predictability. Sheogorath tasks you with saving the Shivering Isles from this horrible fate. One of the main attractions of the Shivering Isles is the fascinating characters you'll meet. The Realm of Madness is split into two worlds: Mania and Dementia. In Mania, you'll find mad artists and thinkers who would certainly love to spend an afternoon high as hell on shrooms and acid. In Dementia, you'll find folks less pleased with life (e.g. a blacksmith who spends more time cutting herself than forging armor). Sheogorath is just as interesting as the rest, although not nearly as insane as some of the residents of Mania and Dementia, he is nearly always contradicting himself and can't seem to control the volume of his voice. One of my favorite characters in the game is Haskill, Sheogorath's dry, sarcastic servant who seems to think of everyone around him as a complete idiot. It's always fum to leave a conversation and see what sarcastic remark he'll make about you. However, don't think that the only reason to get this game is to meet new people.
The main storyline of the Shivering Isles is fairly short (at least compared to the Oblivion storyline), but it is exceedingly fun. You'll meet a handful of new enemies, find some awesome new equipment, meet with completely insane citizens, and generally just enjoy yourself. Not to mention the fact that you'll certainly be entirely satisfied with the ending of the storyline, which offers a great reward.
And, in addition to all the new weapons and armor and enemies and all, you'll also get to do a lot more miscellaneous quests with great rewards. You'll also just enjoy exploring this rich vibrant world full of strange and unexpected surprises.
Although the Shivering Isles doesn't add a terribly large amount of content to the game, it makes of for a minor lack of quantity with superb quality gameplay. The Shivering Isles adds a whole new experience you'll not soon forget.