Shivering Isles was, no, IS amazing. The sheer size of the island is surprising at first, as Bethesda says, about 1/4 the size of Cyrodill.
After completing the SI main quest line, and getting a total of 1250 Gamerscore in Oblivion, I found it took me around 72 hours to total 100%, with about 2-5 hours of messing around. SI has definetely rekindled my Oblivion fondness and I will put another 30-40 hours into it before retiring my copy to a friend or Gamestop.
If you like oblivion, get this expansion.