One of the most broken and monotonous RPG's of recent memory.
First off all, the graphics and sound are very good, and combat feels nice and frantic. Oblivion features a large game world, and hundreds of hours of gameplay...
...and that's where the good stuff ends. The 'hundreds of hours of gameplay' almost exclusively consists of roaming dark dungeons that consist of the same textures, and most of the dungeons consist of copied segments from other dungeons. After the first dozen or so dungeons this becomes oh so not fun. Worse yet, there is a huge disparity between the amount of equipment there is to find and the sheer number of dungeons in the game. You won't finish a tenth of the dungeons in the game before the rest have pretty much nothing to offer you, making the dungeon crawling as pointless as it is boring and repetitive.
With the exception of the too short main quest, the quests feel mundane and unimportant --and seem to serve more as filler than as material. A longer main quest and fewer meaningless side quests would have made Oblivion a *much* better game.
Even worse is the monster leveling system which scales all the in game enemies to your level. In other words, if you are level 1, so is the "Grand Champion of The Arena" and if your level 99, all the bandits and highwaymen will also be level 99.... good bye immersion. Even worse, if your character is not centered on combat, all of your enemies will still scale to your level and will hugely overpower you. There are mods that give enemies a more static level, but as Oblivion was not originally designed with static levels in mind, these mods ruin both the gameplay balance and the pace of the game.
Last but not least, the game is chock full of bugs. I get random crashes to desktop all of the time, and sometimes I will get a crash when I attempt to save... ruining that savegame file. Oblivion also crashes every first time I attempt to run it, and I can only continue my game after my second attempt at running the game.
The Bottom Line
If you want to play a good RPG with a solid storyline, try Gothic. Oblivion is a giant of a game that fails due to gaping gameplay holes, the lack of an intricate and focused main quest, and the pure mind boggling repetitiveness of the thing.