Beautiful, fun, and addictive. Oblivion is everything you want in a RPG and more.
The story line is probably the best in a game I've played in a couple of years. Having just beat it I can say that it never gets old and there's a few big surprises that you would never expect. But why ruin it for ya? I'll just say it's a great compelling story that you'll love no matter what.
You also don't have to play the story at all. You can play however you want. Be a thief and rob people right under their noses as they sleep. Be a warrior and go through all the dungeons or join the fighters guild. Be a mage and join the mages guild and learn magic. Heck, be an alchemist and collect plants and make potions to sell for money. You can even end up investing your gold in a shop at a certain point. There are so many possibilities.
This game will never get old. I'm playing on one character and I'm still not done with ALL the quests. Not only that but there are mods becoming available everyday to make new quests, new places, and new races. It's a game you'll never get tired of and should be on your must buy list.