Expansion pack for Oblivion = Awesomeness
Shivering Isles doesn't add anything new to Cyrodiil, rather, a strange Door appears in Niben Bay, a portal of sorts.... a Door to Lord Sheogorath's Plane of Oblivion, the Shivering Isles, divided in the lands of Mania and Dementia. Which represent the light and dark sides of Sheogorath's madness, respectively.
It's a completely different place from Tamriel, a surreal land, full of Bizarre Creatures, just like its inhabitants, who are all mad or paranoid... or both, just take a look at the people in the Daedric Shrine of Sheogorath and you'll get the idea of how the citizens of the Shivering Isles are. And of course, the Prince of Madness himself, Sheogorath, doesn't have a bit of Sanity in him... CHEESE FOR EVERYONE! Oh wait, scrap that, cheese for no one. probably the only "sane" character is Sheogorath's assistant, Haskill.
There's a good amount of new items to keep collectors busy for a long while, as well as a good amount of side quests from the demented citizens of the Shivering Isles, as well as the SI Main quest, which is all about stopping the Gray March, and invasion of the Knights of the Order, under the command of Jyggalag, the Daedric Prince of Order, whom Sheogorath finds to be boring.
The expansion has a lot of new content, and is a completely different experience from the rest of the game. There's a good amount of new items, like Bone Arrows, Golden Saint and Dark Seducer Weapons and Armor, Grummite weapons, not to mention you can get Weapons and Armor forged for you if you bring Amber or Madness Ore to the Smiths in the city of New Sheoth. There's of course new enemies, like Grummites, Hungers, Elytras, Gnarls, Flesh Atronachs, among some others. And, well... you get the idea.
Overall, if you enjoyed Oblivion and completed every quest there is and you were left desiring even more, you should totally buy the Shivering Isles, you'll definitively love it.