The folks at CVG have written up their own review of The Elder Scrolls IV: Shivering Isles, giving the expansion pack an overall score of 7.5/10.
You're going to be doing a hell of a lot of hacking here, and if you're happy with that then you'll get more than your money's worth.
If not, join me in a chorus of scorn on great designers wasting their talent and success. Leave 'good' to the EverQuests of this world, Mr Bethesda Softworks. Blow us away next time, and you can have another shiny Must Buy award. For now, this one goes back in the drawer.