Oblivion is simply one of the best games I've played, you will easily get 50-100 hours of gameplay. Visually it is beautiful but has a few flaws, there are hundreds, if not thousands, of NPCs but there weren't too may voice actors hired and you may start to become tired of them. Saying that though, you can talk to every NPC in the game so I suppose its a small price to pay. Also some of the main voice talent in the game include Patrick Stewart, Sean Bean and Terence Stamp.
The Game of the Year Edition comes with the two main expansion packs on a 2nd disc. You will need a HDD to play them, as they are installed to it. After you install them to the HDD you never have to use that disc again. Oblivion does have other DLC but they are not included in this set, they need to be purchased though Xbox Live with microsoft points.
Oblivion has 1250 achievement points and are earned by completing the main quest, the arena, the guilds and the Shivering Isles expansion. They are relatively easy to get but just take a long time. I believe it took me 78 hours to get the full 1250 points, and I'd completed the game on the PC beforehand (I don't skip dialogue though). Loading times may be an issue for some, especially in the outdoor areas but the game world is massive and again its a small price to pay.
There are many different ways to play the game, you can fight with a sword, bow and arrow or with magic. Your character has a vast choice of Armour and weapons available, especially when you level up your character. Overall Oblivion has a few flaws but in such a large game it doesn't ruin the experience.