Oblivion was a true masterpiece, and shivering isles is great but compared to oblivion it doesnt really make the grade
The enemies themselves also seem i little more intelligent. In the regular game, time and time again would the enemy just run at you whiclst you back-peddled whilst firing a spell at their face. In shivering isles i am sure that the AI respond differently (It might just be luck, but i have been flanked before which never happens in the normal game).
There is one aspect of this expansion, however, that lets it down when compared to the main game of oblivion. Its that the characters are just, well, plain stupid. i know its the realm of madness, but seeing a guy wandering about moaning about not having a particualr fork is funny for the first two or three times but then it losses its comedic grace and becomes an annoyance. Sheogorath himself has a few one liners but in the long run you just end up skipping everything he says because he says it so slowly and so painfully imaturely. Humour is a fine but in this game it comes at the expense of some of Oblivions epic qualities. Its less of a high fantasy world and more of a fairytale kingdom.
While a majority of the enemies are nice additions, the main enemy - the knights of order - are not as compelling or as 'cool' as Dremora's, and they are not accompanied by an array of bloodthirsty demons like Daedroths and atronarchs, so the combat in the main quest can become a little repetitive. This aside, theres enough new content in this game that allows you to ignore the games minor short falls. after all, its what you would expect from a game entitled "Elder Scrolls", isnt it?