This is more than enough to get you back to Oblivion.
We all know Oblivion's flaws - the constant freezing, some annoying bugs and the fact it reaches an end eventually - and with this expansion pack one of the flaws can be postponed. So anyone who liked Oblivion should have this, no matter how, especially since the game of the year's edition is being shipped with both expansions - Shivering Isles and Knights of the Nine - which makes it an even more essential deal. You can also purchase at the fine Xbox Live Arcade or at a store in a physical purchase. No matter how you get it's worth because who wouldn't add a 20-30 hours of gameplay to the already gigantic and awesome Oblivion? Certainly not anyone who liked the game, that's for sure. Not even counting this expansion pack is longer than most games today.
So, you install the pack - either by downloading, buying the physical release or by the secondary disk of the special edition - start the original game, load your character, doesn't matter which one it is since you can experience Shivering Isles with a level 40 as well as a level 5 character, not mattering how advanced you are in the original game or how many tasks you have completed, wait a while on your game doing something or travel to somewhere to pass the time and you'll receive a new message task, the message tells you that there's a problem with a mysterious door located somewhere in Tamriel. Make it your current objective and head there. Arriving at the location you're gonna hear stories about the world inside it, and of course at this point you eager to find out more about it. You enter this new world.
It goes like this, the leader of these lands is the Daedric Prince of Madness, called Sheogorath - the Madgod - he's the big shot around there. He rules a land divided in two, the light-colored Mania and the dark-colored Dementia. Each has its own duke and duchess and they don't get along very well, to a point of having two different places in the city, each for one kind of people; the rest of the land is also divided, into Crucible and Bliss, following the characteristics of each populace. This divided land is facing problems with an event known as Greymarch, this apocalyptical occurrence happens periodically since long ago but this time the Madgod plans to not just stand there with his arms crossed. The Greymarch is when another Dadric Prince, the Daedric Prince of Order, invades the land and destroys everything. It's up to you to help the Madgod and stop him.
As well as in Oblivion you choose if you wanna focus on the main story or not, there are many other tasks to be done in Shivering Isles, many related to the subject of divided population, but also item hunting, assassinations, and all that. The main story shows a slight change of direction depending on which choice you make in certain situations, the choice has to do with the two distinct sides and even though the rest of the game and tasks continue the same and available no matter which choice you make, the main storyline suffers slight differences and rewards you with distinct items based on what decision you've made. As you advance helping the Madgod you get promoted to higher levels until you finally reach the highest point, as his successor. This works much like the guilds in the main game, where you also go climbing in position and gaining respect of the others. There's also additions like new weapons, types of armors, creatures, ingredients.
Shivering Isles is actually a parallel world to Tamriel. Even though the map of this expansion isn't as big as the main game's, it's still very big, with many locations scattered around and with many tasks to attend. Visually it has its own style but overall it looks pretty much like the original technically speaking, so no graphical enhancement can be noted. Actually the presentation is pretty much Oblivion, what shines here is the opportunity to play this amazing game again with multiple brand new events and a new main story. And this time the main story plays a much bigger role, meaning that most of side-quest are somewhat related to what's going with the world of Shivering Isles - actually more the inner war between the two sides than the actual threat of doomsday.
If you still are not sure about this you should ask yourself a very simple question. Ask "Did I like Oblivion?". If the answer to that question is yes then you absolutely should get this, because it uses Oblivion's formula and game mechanics to offer an extent of gameplay that's surely worth the purchase. If you didn't like Oblivion then you should go look for a more suitable game for yourself. It's quite simple really. The game overall manages to maintain almost the same level of awesomeness as Oblivion delivered, with a nice new main story, new tasks, new dungeons, new items, new locations. Everything will seem fresh, though the system will be familiar enough. A great motive to re-experience this amazing RPG experience.