Oblivion is no doubt a great game.
There are many different quests and groups you can join. Among some of these are the standard fighters guild, mages guild, blades, thieves guild, and the assasigns guild. Each offer a variety of quests and missions.
There is a new feature in Oblivion that allows u to "fast-travel". You do this by opening up your map and simply clicking on a location you have previously visited. I found this to be a usefull tool, but could have it been to useful? I think the fact that you can travel great distance in a simple click with no fees or such takes away from the game. The "fast - travel" option for me was to much. There should of been something like "stilt-striders" from morrowind for travelling.
The game itself is decent in length. If you stick to just the main storyline you would probably get a good few months of gameplay before you can beat it. But that is also dependant upon how often you play the game. I played the game everyday all day and it took me a few months.
Another downside alongside the "fast travel" is that the game likes to be choppy during battles. I dont know if its just from my computer not having enough "umphf" but i have it rigged pritty nicely and it would be choppy during fighting, in the outdoors. But while inside a cave or dungeon all of the gameplay seems to run smoothly.
I thought the ending of the main quest was kinda dumb. It was a cool scene and all but it just didnt wow me enough.
After beating the main quest i didnt really feel the need to play anymore and i havent played it since. So i dont think the replay value is really good. Unless you want to play and go through all of the guilds and groups and what not.