Oblivion is a massive game. I'm talking hundreds of hours of content for you to happily wade through with nary a care for the outside world. Heck, you can just spend a whole day stealing apples if you want, and that's before you even bother with a side quest. So one has to applaud Bethesda for giving gamers value for money, and they've done so once again with The Shivering Isles, which is perhaps the best expansion pack to ever be released for a game.

First of all, this expansion is only available at the moment through download over Xbox Live, and it will set you back around $40 in Microsoft Points. There is, however, rumour of a Game of The Year addition, so those of you who covet physical discs for their expansions need not fear, a copy that occupies real space should be available in the near future.

Part of the beauty of this expansion is that it can be enjoyed regardless of your level, so it's just as fun to start up a brand new character as it is to go in with your uber-leveled one. In fact, a lot of the stuff that you'll gain will probably be most enjoyed with a new character, as there's a kind of hubris that goes with having a character above level 25 that sees you sticking your nose up at most new spells, weapons and armour.

There is so much extra stuff in the Shivering Isles that you'd have to be in a coma to not be impressed with what's on offer. Bethesda apparently started work on this expansion directly after shipping Oblivion, and this is said to be the only expansion to the game – so it's fitting that the new armour sets, creatures, plants, weapons and spells are so darn cool that the whole game feels almost new again.

The Shivering Isles works as a stand-alone area, introducing the realm of Sheogorath, a genuinely crazy immortal with a penchant for screaming out that he'd like to "pluck out your eyes”. Your portal to this realm is a strange door that appears in Niben Bay and let me say that the very first unveiling of the new area is both thrilling and unexpected, and acts to set the level of quality for the entire expansion. The starting area itself immediately feels different to the forest environs of Oblivion, with extremely detailed ruins and twisting black trunks that end in large, fanning mushrooms, as well as the beginning town of Passwall, which reflects very strongly the architecture of Morrowind.

Missions are also tightly scripted, but in a good way. Remember those awesome Dark Brotherhood quests? Well, pretty much every quest in The Shivering Isles is of that quality (coming as they do from the same mind responsible for the nefarious DB quests). In fact, I'd even warn you that if you want to keep playing as an entirely good character then that may not actually be possible in this expansion. Several missions see you choosing between two evil outcomes, which to my own demented mind was a dream come true. You may even find your own maniacal laugh mirroring that of Sheogorath's on several occasions.

So what else is new? Well apart from the fact that the isle is split between mania and dementia – each with their own artistic feel – there are a heap of new plants and alchemical ingredients, all of which are compatible with those from Oblivion – so all you test tube jugglers will be happy. New weapons also pop up, a particular favourite of mine being a rather large sword that keeps count of how many kills you make each day. There are also two new major armour types: amber and madness ore provide the material for both of these and can be purchased from particular weapon smiths throughout the isles. You'll also come across several matrices, which allow you to transform amber or madness ore into new weapons. All of these look appropriately cool and are of course incredibly detailed but will likely only be worth pursuing for low level players who aren't quite so attached to their helm of intelligence of some such awesome piece of kit.

You'll also gain access to either Golden Saint or Dark Seducer armour, depending on your decisions throughout the game. This is the real strength of The Shivering Isles: its ability to give you choices and make you feel like you're actually part of a changing and evolving world. This starts with your killing of the Gatekeeper, a magical monster that guards the gates to the Isles – and after this fact you're constantly hounded by people angry that the damn thing's been slain. This carries through to a later quest where you'll feel compelled to make up for your previous action(s).

Most mission follow this template – giving you real-world consequences for your actions, and it's something that perhaps should have been done to more effect in Oblivion, but at least they've managed to bring something new and exciting to the world with this expansion. Because of this, there's always an impetus to complete a mission, even if behind all of that great dialogue and those cool set pieces there's a generic fetch quest at hand. You really do want to find out about the Greymarch and Jyggalag, so it's a testament to the creative department that The Shivering Isles actually feel more alive and, well, interesting that most of Oblivion.

It also needs to be said that the quality and quantity on offer with this expansion is as good, if not better, than most full releases. You're definitely in for a good 30 hours of questing here, which is significantly more than most games at full retail can claim. In actual fact, The Shivering Isles is so beautifully executed that you'll fall in love with the game all over again, and that's gotta be priceless.