To be honest the first time I tried to play Oblivion I didn't like it, so I ended up getting rid of it, but then a year or more down the road I started thinking about it and wishing that I had it, so I went out and rebought it. All I can say is that I'm so glad I did, because after playing thru the whole game ,even without it's DLC, it was amazing. Everything you get to do, the size of the world, side quests and how fun it is to roam around doing anything you want without any strains to hold you back, is remarkable, games these days don't offer this kind of freedom, and that's what makes Oblivion so special. There are a lot of other things that make this game special like its leveling up system and its different classes. Having said that though ,there are things that I thought Oblivion could have done better, like its combat, it's not bad or anything it's just that it's a bit clunky, and its story, it's an okay story, it's just that it would have been nice if you would have played a much more important part of it, for instance it would have been nice if you could've become king in the end or if you could've made some choice making options along the road or something like that. Despite some of the things I wish the game would've had, I am very content with what it has to offer. I wont say much about the DLC other then I enjoyed Knights Of The Nine but was a bit disappointed with it, and that I really enjoyed The Shivering Isles other then some of its dragging moments here and there, or some of its blah dungeons and caves. Overall all on its own Oblivion is a truly amazing game and an achievement at the least, and the truth be told it doesn't even need any DLC to make it better, because it already has more then plenty of content all together, but I will say this, the DLC does add to the appeal, giving you more out of your money's worth. If you love RPGs and cant get enough of them and you haven't played Oblivion yet, then you should, because its one of the standout games of this and the previous generation.
