The Good: 30 hours of game play, tons and tons of new content, better story, great new quests, new art style is wonderful
The Bad: After you finish everything you'll want more, feels too much like Oblivion
I enjoyed Shivering Isles a lot more than Oblivion (not saying I didn't enjoy Oblivion!) and there's just so much here! First off, the story is wonderful, exciting and better than Oblivion's. You enter Shivering Isles via a strange gate just out side the Imperial City. After entering and talking to the people who've apparently gone mad you find the realm's creator, Sheogorath, and help him stop the Greymarch and the Order of the Knights who are trying to destroy his realm. Underneath all this two factions known as Mania and Dementia (basically insane and REALLY insane!) are battling it out. While you help Sheogorath you can change the storyline by deciding which faction to help out and join. This makes you want to play through twice! While Shivering Isles is about 25% the size of Cyrodill you always feel like it's bigger in some ways. There tons of side quests and lots of main quests to complete with hundreds of new items, tons of new enemies and characters, as well as more dungeons and places to loot. There are three main towns you can do things in, Passwall (which is before you head deeper into the Isles), Crucible (which is the town of Dementia), and Bliss (the town of Mania). You can forge new types of armor using Madness Ores (heavy armor), and/or amber (light armor).
Another thing I need to mention is the new art style for Shivering Isles. Everything is twisted and deformed like an insane person's mind. The new style is great and gives the graphics engine a shiny new touch. There's just so much here you'll stay busy for hours and hours and its well worth your $30. You may not feel that way until you delve deeper into the game but I promise you Shivering Isles is well worth you hard drive space and money.