It's good in some ways, but severely lacking in others. Graphics-wise, you need a high-end machine to make it look decent. There are piles of quests in the game, but once you're done them, I have a feeling the game world will prove to be rather uninteresting to explore.
The modding community is massive, though, so there's really no reason to run out of things to do. You can also ignore the main quest and side quests altogether, and do whatever you want. Being a thief is VERY fun. You can also run around, just randomly killing people. <img src="style_emoticons/<#EMO_DIR#>/laugh.gif" style="vertical-align:middle" emoid="

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On this computer, combat is very tough, as my character takes forever to react, and gets himself killed often - this is with a mid-range character, too, so I think it's the computer causing this.
I don't know what the official add-ons are like, though, as I've only got the stock game. Apparently, the "Game of the Year" Edition has the entire game bundled in one box.
The Oblivion Wiki would probably be a good place to look if you're curious about the quests, factions, and other stuff, but this will spoil parts of the game if you do decide to get it: <a href="http://www.uesp.net/wiki/Oblivion:Oblivion" target="_blank">http://www.uesp.net/wiki/Oblivion:Oblivion</a>