Excellent! New creatures, new ingredients for alchemy and interesting personalities.
I am at 25 level and just entered The Shivering Isles. In some ways it is different and in others it is not. The landscape looks different from Tamriel in some ways and looks like Morrowind in others. The first think I noticed was the plants either blend into the scenery or jump out at you screaming with color. Being a Mage I immediately started collection new ingredients for my potion making.
I then started running into the new creatures of the land. I have found them interesting to look at, but only a little more interesting than what I have seen in Tamriel. There are some returning creatures, Golden Saints (which are one of the two guards in The Shivering Isles, so don't attack unless you want to go to prison) and Hungers from Morrowwind. The creatures look interesting, but as I have a stealth of 100 and high camouflage, they are not that different to fight than anything else. There is one (I can't remember it's name) that I think would be a real pain to fight. It would occasionally see me as I was casting spells from a safe distance, then it would sort of blur and streak across the landscape and end up right next to me. I mean it was FAST!!! I would hate to try to outrun this thing (if you even could).
The main city is pretty small, with two sections dedicated to the two sides of The Shivering Isles. Both sections have shops, an armorer (where you have custom armor made with the right materials), homes, homeless and inns. Interesting, but quickly covered. So far (and I am just starting in the Isles), it is a fresh addition to Oblivion with some new things to look at and collect. For fans, this is a definite buy. If you have "been there, done that" then maybe not (the only really different thing is the personalities you will meet, they are much more varied and interesting that in Tamriel).