Its like being invited to tea by the Mad Hatter and then finding out he needs your help to save Wonderland.
Its easy enough to access the new land, since a huge doorway/portal appears in one of the southern lakes in the original "Oblivion' game. Once there, you're tasked with getting past the large wall and gates to properly enter the Isles, and your eventual meeting with the Madgod himself.
The Isles are divided into the realms of Mania and Dementia (like most real world homes). Each realm has a unique feel and appearance, though they are both ruled by Sheogorath, for the most part.
The main questline is definitely a lot more interesting than many of the quests in "vanilla" Oblivion. (you know what they say, once you go mad, you'll never go back!). The first quest on the main questline, after meeting Sheogorath, especially, is a lot of fun and its a shame that it isnt repeatable.
Outside of the main quests, there are quite a few miscellaneous quests as well, from people in towns as well as little hamlets out in the middle of nowhere.\
You'll run into quite a few colorful characters as you explore the Isles, but none is as interesting as Sheogorath himself. He's all at once hilarious and annoying, and not a slave to fashion either, like your modern princes of madness.
The graphics are a lot more interesting than Oblivion as well. Both Mania and Dementia have their own sides, just as the Isles themselves do. Whatever you look at, looks pretty darn good though, from the characters to the Isles themselves.
The voice acting is really well done also, even Sheogorath, who, lets face it, sounds like a drunk leprechaun at times, (and makes about as much sense). Most of the main characters sound like you'd expect, though.
Not to spoil anything, but there a whole lot to do, in and out of the main questline, and a whole lot to see, wherever those quests take you. If you've had your fill of Oblivion and need a vacation from Cyrodil, you could do a lot worse than the Isles.