Oblivion GOTY is even better than one of the best games this gen. Pure genius.
Gameplay- Oblivion has sooooo much gameplay. You can dungeon crawl, become an assassin, an arena champion, help people out in town, become a successful businessman, etc etc etc. You can basically do whatever you want in this game. Join a guild, loot caves, become an outlaw, or just covertly kill people int he middle of day. The combat is great.
Graphics/Sound- Both are pretty good. The character models are okay by todays standards, but its the world that looks great. The things you can see in the world of Tamriel are astounding. Just go to the top of the mountains in Bruma, and look at the landscape of the world. The sound is good. The music is great, and the voice actors are good.
Fun/Value- This game is so fun. I had hundreds of hours playing as everything from a mage to an assassin to a mighty warrior. There is so much stuff to do in this game. I is one of the few games I would spend 100 on and feel completely satisfied.
Bottom line, if you like rpgs and want your money's worth, get Oblivion GOTY.