Gigantic and epic. That is another great work from Bethesda, a great pick-and-choose RPG-like game.
It is certain that if you would spend your whole life playing Oblivion, you would not uncover all of its mysteries. You could easily play this game for days, weeks or even, hardly, months and still your hunt would be incomplete, although I will admit that this game has almost no difficulty, making it seem as if you had cheated from the beginning...
The game's physics are another quality that must be taken as a significant one, knowing the lack of interaction in most RPG-style games. But one big flaw I have seen and have not liked. That is your lack of choice in the main quest, which makes the game's ending pretty obvious and "oblivious". However, this flaw has almost no effect upon Oblivion soil, leaving the game to the classics list.