Summary: As far as DLC goes, Shivering Isles is a prime example of DLC done right. It is curious how Bethesda's horse armour was a great example of how NOT to do it, and SI is the opposite. Like with Bloodmoon, the pack offers an entire new island which, in my opinion, surpasses the original game in terms of environment design because everything in Oblivion looked exactly the ruddy same.
Summary: In many respects superior to the main Oblivion game (possibly a case of over-familiarity in the case of the latter); Shivering Isles is huge, features excellent writing, fun and varied quests, improved voice-acting and best of all? The locales and environment are highly reminiscent of those in Morrowind - I had severe bouts of nostalgia exploring the settlements and wilderness areas of the island.