Entertaining for the duration of the main quest, but not much else will keep you interested afterwards.
The gameplay mechanics work fairly well, although it may take some time for a user to be fully capable of using the player menus to its full potential. That being said you don't need to learn how to use it all in order to complete the game. Just equip the desired item and use it. Weapons all move fairly realistically, and there are quite a few weapons in the game, from the ShortSword to the Elven Battle Axe. Everything just seems to work well, so you won't find yourself fumbling at the keyboard very often.
The graphics for the most part are very good, but not everyone can view them at full settings because of the heavy performance hit. HDR is put to use, but can sometimes seem over done, and you may want to leave it off for a performance increase. Character animations and models are of high quality, but are let down somewhat by the textures which seem a bit bland at times.
You will encounter some magnificent views in Oblivion, like looking out over a lake while mountains are drenched in the glow of the setting sun. And for the most part, they are quite breath-taking. But as I've said, you'll need a beefy system for the full effect.
The sound in this game is top notch. The cries of battle, the clanging of metal all do a great job of immersing the player. This really drives up the tension in some situations. The music is also very well done, but may be hard to notice at times when you are distracted with tasks. If you're travelling the hill-side, you'll notice some very pleasant and subtle tracks, while in battle your chest will pound with the intense drums of an orchestra.
Overall, this is a good games that will please many fans of the genre, but after the main quest you won't find much else to keep you interested. There is also no multi-player, which is a shame. It would have been fun to have a battle with friends in the Challenge Arena.
If you like The Elder Scrolls then it's worth the buy if you have the time for it.