Shivering Isles may not add a great deal in terms of something different to Oblivion, but hey it's Oblivion...
Shivering Isles cast as I mentioned before, is also one of it's stand out points. Sheogorath makes for a colourful lead, leaping about merrily and madly and greeting you with lines about unwinding your intestines and the like. Other prominent cast members worth mentioning are Syl the Duchess of Dementia whose unending paranoia sends you on a rather interesting set of quests, Hasgill the ever polite but mildly sarcastic butler to Sheogorath and the palace guards themselves who are divided into two factions who dislike each other leading to some interesting stories and dialogue. Something Oblivion lacked was a memorable cast and short of Martin and Jauffre they are all instantly forgettable, Shivering Isles does something to remedy this and it makes the game far more enjoyable as a result.
Perhaps the aspect of Shivering Isles I enjoyed the most was the side-quests, it features some highly entertaining ones one of which involving you helping a man kill himself while it still being a surprise to him AND making it look like an accident. It took a great deal of though to get this quest complete and it was a lot of fun actually achieving completion of it too. Shivering Isles most certainly features some of the most innovative sidequests to grace an RPG and are well worth taking a break from the achivement ridden main plot to investigate.
There are certainly some complaints to be levered at Shivering Isles too though, some borderline game breaking bugs have still survived unpatched and it took me 10 goes to actually get the games main cities to load without crashing (a commonly reported bug when the game launched). When it did finally work, the game didn't crash again, but it can be majorly off-putting to crash the first time you try and enter the place where the games main plot kicks off. The frame-rate in general seemed to be choppier once I got to Shivering Isles too. While the original wasn't perfect the slowdown in Shivering Isles was painfully noticeable at times and something that still needs fixing.
Also while Shivering Isles does add plenty of new quests, some of them quite brilliant it still is essentially "more of the same" and if you played all the way through Oblivion you may be sick of the game by now and not enjoy this at all. While it was undoubtedly a great expansion I am certainly all Oblivioned out after playing Shivering Isles and I'm glad my month and a half long trek through the Elder Scrolls IV and all it's expansions is at an end.
Oh and finally what idiot thought it was a good idea to make you play (or at least reload a saved game) Shivering Isles twice to get all the 360 achievements??? I may be an RPG fanatic but I'm certainly not a masochist.
As you can probably tell despite some bugs and me suffering from Oblivion burnout Shivering Isles is fantastic and if you liked Oblivion your sure to love this, just maybe space out playing it a bit more than I did!