ES4: Shivering Isles - Reviews @ GameSpot, GameSpy

GameSpot has kicked up their review of Shivering Isles.  The score is 8.6/10 and here's the opening:

As if to answer the decades-old query posed by Iron Maiden in the single "Can I Play With Madness," Bethesda Softworks brings you The Elder Scrolls IV: Shivering Isles, an expansion in which you most definitely can play with madness. Shivering Isles is an expansion to The Elder Scrolls IV: Oblivion and introduces a new world along with a lengthy major quest. It also includes several minor quests, new spells and items, and dozens of completely insane characters with which to interact. The theme of the new world is madness, which is manifested in the batty characters and the twisted yet beautiful landscape. Aside from the new realm, there isn't anything in this expansion that will significantly alter or enhance your experience with the rest of the game. So if you're content with Oblivion, you won't miss out on any integral updates if you decide to pass on Shivering Isles. However, even though Shivering Isles isn't a must-have expansion, it's an excellent value based on the amount of content alone, and a great excuse to revisit an already excellent game.

...and GameSpy's review has a score of 4/5:

This main storyline is Shivering Isles' greatest strength. In terms of sheer complexity, it easily rivals the best quests of the original game and it far surpasses them in sheer fun. Early on in the main quest, Lord Sheogorath asks the player to reactivate one of his favorite "recreational sites." This turns out to be an elaborate deathtrap for adventurers in which the mad god can watch them run through his maze and push buttons that either drive them insane or outright kill them. Naturally the player also gets a chance to do this with a poor unfortunate party once the area is repaired, and it marked some of my most enjoyable experiences in the game.