It's a neatly executed game - the graphics look really smooth but the environments look detailed. Character models look terrible to be honest - all curved and "piggy" looking. They look all round and chubby. The bad guys look rushed but the boss fights are quite epic in scale and visual content.
The campaign is almost infinite. I've probably spent a good 70 hours on the game but I'm nowhere near the end. Thats one of the bad and good points of the game. The good side of it is that for the price you pay, you're getting something so huge but the bad is that its so long that you just get bored of it since the campaign is frustrating. Yes - the story is good but since there seems to be no end, you just get so fed up of playing again and again since its almost as if its not getting you anywhere. Eventually, you should get to a point where the story seems to move forward but you're probably not gonna bother even getting past the first 30 or so quests. You'd probably be distracted by the several, innovative side quests and the extras you can do in each town/village etc.
It's got a true sense of freedom to its world but once again - cheap deaths, such as dieing by goblins (no health potions to use).
The soundtrack is rather epic and deserves special mention. The speech/interaction system is also a welcome feature - vastly improved upon compared to Morrowind, but some may not like the very archaic and lengthy dialogues with practically lifeless character models.
All in all, Oblivion can be what you want it to be. A good game but you'll easily get bored of it if you don't like committing yourself to a game.