Oblivion has something for everybody. Simply put it redefines open ended RPG's on every level.
I admit that when I first picked up Morrowind I did not like it within the first five minutes. I stopped playing and didn't pick it up again until about 4 months later. The second time around it grew on me.
Oblivion takes every aspect of it's predecessor that fans loved, and bumps it up a notch. From beautiful landscapes, to realistic and fast paced battles, Oblivion has a little bit for everyone.
Graphics: Being one of the first games released on the PS3, Oblivion has eye popping graphics. I personally found myself on more than one occasion stopping game play just to simply look at the scenery. You will see calmly shaded meadows, snowy mountain trails, burning wastelands, moonlit forests, and much much more while playing Oblivion.
Gameplay: If your new to the Elder Scrolls series this might throw you for a loop. This RPG is designed to be played from the first person, although you have the option of being in third person (I personally find it much more difficult to play in third person). It takes some getting used to, but a well designed control scheme will allow you to get the hang of it in no time. Another very nice feature of the gameplay is the simple fact that you can interact with almost ANYTHING in the environment. If you want to punch someone in the face, you can. If you want to break into someones house and rob them blind...you can. There are very few limitations.
There is also much to be said about leveling in Oblivion. You gain levels by improving your major skills, which you yourself pick based on your class (which you can choose or create). These major skills cover just about anything you will be doing in the world of Oblivion. For example acrobatics is increased by jumping on/off of things such as roofs, rocks, people (if your really good). I'll save a detailed description of all the skills for one of the many walkthroughs out there, but lets just say that the options are designed to fit anyones needs, whether you want to shoot enemies from afar (marksman), cast powerful spells (mysticism, alteration, etc.), or simply beat down your enemies with your bare hands (hand to hand combat).
Storyline: I'd rather not spend too much time on this, because the story in Oblivion is what you make of it. Since you ultimately decide the fate of your character, the story can be altered at almost any point in the game. I personally found the story to be just a means for enjoying an already awesome game, and as a result got a kick out of completing the quests in the main plot.
Sound: Probably something that Oblivion lacks a bit on. I'm a big fan of background music and there were times when playing Oblivion where I just felt it was awkward silence. And the musical score is nothing to brag about. The battle music seems a bit bland and repetitive, but that's coming from a guy who listens to Final Fantasy soundtracks on his way to work.
Lasting Value/Summary (buy or rent this game?): Oblivion is overall a great game, and the kind of game that you will pour many hours into discovering its many secrets. I left a lot of the amazing details out about this game (such as the different factions you can join) for good reason. I think anyone can enjoy this game, especially considering I started out not enjoying the series (as I explained above, I hated Morrowind the first time I played it).
This is definitely a game to BUY, not to rent. Games like Devil May Cry, and Assassins Creed can be bought, beaten, and returned with no regrets. Oblivion however is a game that you will want to continue playing for many months to come.