ES4: Shivering Isles - Reviews @ Boomtown, Worthplaying

Shivering Isles has been reviewed at Boomtown with a score of 8/10:

I personally think that Shivering Isles is a more interesting place than most of the regular Oblivion, and it's a joy to see what waits around the next corner. The official roll call of new content says the expansion features 32 new ingredients, 10 new monsters, 9,500 lines of dialogue, 200 new weapons and 80 kinds of armours and clothes.

To cut to the chase, the expansion is worth all the money for all fans of the regular game. Many of the fundamental problems haven't been solved, but honestly, this game has always been so impressive to me that I can live with the smaller bugs. And Shivering Isles with its mad content is basically an added spice to a dish worthy of several Michelin stars.

...and Worthplaying is next up with a score a 8.5/10:

Without disclosing too much of the storyline, I will say that you won't simply teleport to Shivering Isles; you will first be challenged to prove your worthiness for the task of becoming the mad king's champion. You will need to figure out how to defeat the Gatekeeper of Shivering Isles to obtain the two keys to the Gates of Madness. After successfully defeating the gatekeeper and entering the main portion of the Island, you will need to meet with your new employer (Sheogorath) and assist him with a series of quests. Some of these quests will challenge you to explore your own morality (reminiscent of the classic Ultima series), and other quests will simply be errands to fetch various items.