ESIV:Oblivion could easily be one of the best RPGs to ever hit the market.
The Elder Scrolls series, from its inception in the early 90s in the form of a game called "Elder Scrolls: The Arena," the game world attempted to be one of the most realistic and immersive of any game at the time. While each version of this series (save Oblivion) has had its share of bugs, and flat out overwhelming non-linearity, Oblivion has done something really special for this series.
Let me start off by saying I've found it extremely difficult to find something negative about this game, as everything (after 40 hours of playing) seems to be a shining example of gaming perfection.
Graphically, Oblivion is easily the best looking game on the 360 (or any system for that matter) and will really show off what the system can do. Whether it is a blade of grass swaying with the wind or being trampled to the ground as you step upon it, or a randomized cloudscape in the sky slowly making its way across the landscape, there is plenty of immersion to be had in this beautifully made game.
The Audio is a shining point in Oblivion as well. This is the first ES game to have full voice acted NPCs. This is easily one of the best things to happen to the series, as all of the dialogue in the originals can be extremely lengthy (like multiple pages of dialogue). This was prevalent in Morrowind, Oblivion's ancestor, and to many had a negative impact on the game. This situation is gone, and the voice acting talent is some of the best of any game. For one, the cast features the vocal talents of the very recognizable Patrick Stewart (Picard, Star Trek TNG), and Sean Bean (Patriot Games, Boromir from Lord of the Rings). The rest of the cast comes off just as professional, and they add to an all-to-believable immersion that radiates within the game.
As for Gameplay, the Elder Scrolls has always been a top knotch game for overall fun and gameplay. Oblivion is no exception, and easily offers the best gameplay of the series. Thanks in part to its incredible graphics engine, the game can offer some really wicked physics, excellent AI, and some really different quests.
Oblivion, like its previous ES companions, features a deep and intriguing plot that remains clear about where you need to go and what you should be doing. However, this does not equal to Oblivion being "dumbed-down", its just much more refined than others before it. Also like its predecessors, Oblivion provides the opportunity to role play a character however you'd like and do whatever you want at any given time. You can take the quests at your own pace, or literally just go off and do the myriad of side-quests that await the wanderer in you.
The world itself is massive, and as such an auto-travel feature was implemented to make things a bit easier on the player. It should be noted, however, that many towns, hamlets and dungeons need to be discovered before they will be a part of your auto-travel map.
Oblivion offers some of the best replay value in its diverse character creation, and the ability to play that character how you wish. Do you want to play an evil assassin who waits in dark corners for the perfect time to strike their prey? Do you want to become the most powerful Wizard in the world, or perhaps the most famous warrior in the Imperial Arena? Does the idea of joining the elusive Guild of Thieves peak your curiousity? (It should ;) )
While the main quest could probably be beat in about 20-30 hours of play (if you stuck to the main quests only, you're missing out on the best part of the game. This is not a game to be rushed through if you want to get a truly unique experience, and besides you'd be missing out on one of the best games ever made.
To conclude, Oblivion is Game of the Year, hands down. The production values that drove this game are a shining example for the entire industry to follow: you don't need a multiplayer component to make the best game. Oblivion offers so much, even to newcomers of the RPG genre, and shows off the power of the Xbox 360 console. With updates promised for both the PC and 360 versions of this game (content updates through xbox live), Oblivion stands on the edge of near infinite replayability.
There is simply no reason not to get this game on one of its released platforms. If you don't have a powerful PC but you have a 360, then get this version of the game. Its the best looking game on the console for one, and unless you have a REALLY high end PC (minimum of a 6800 series NVidia and its ATI equivalent), the 360 version will look better.
So waste no more time, and go and purchase one of the best games ever made.
That's my take, take it or leave it.