I'm sad and surprised to say I'm a bit disappointed. I loved Oblivion and must have played it through about four times. As with Morrowind. But there's definately something missing from SI. It's just as lavishly detailed as Oblivion and the voices ect are just as well done, and the new land is beautiful. But I found that once the initial novelty of a new realm to explore and new monsters wore off (it was bliss for a couple of hours) I just started to get a bit bored.
The main quest line just isn't that interesting. And I just can't get enthusiastic about working for Sheogorath. Plus some of the quests are distasteful, like murdering someone to assume their title, and torturing people. There's no way my character would do that. But you have no choice because the devs didn't see fit to give you another option. Very few of the side quests had you doing anything you could get enthusiastic about either. Nothing at all like say, the Mazoga the Orc, or the Forlorn Watchman quests that made you really care about the people, so you really wanted to do the quest to help them.
By far the biggest problem though is a serious bug that makes your game unplayable if you didn't install the beta patch BEFORE you started having trouble. I didn't and now my game's ruined. Plus all my saved games with my precious face that I'd worked so hard on to get right are also apparently ruined. So I can't say my experience with Shivering Isles has been much fun.
That aside though, and once they get the bugs worked out it's still worth a play if you're a fan. But back up your saved games before you install it.