Pros:
-Rich lore
-Fun first/third person combat
-Massive weapon, armor & outfit variety
-A very interesting open world with lots to explore & discover
-The music is amazing
-Lots of little random events scattered all across the setting many of which lead to profound experiences. Every replay is different yet there's so much variety I have yet to start a new character.
-The PC version allows for modding. There are a TON of mods which adds hundreds if not thousands of hours of replay value to the game.

Cons:
-Performance. It's pretty hit or miss depending upon your system configuration
-Story. While the lore is rich the main story is rather bland and most of the characters are either annoying are rude/bigoted. Their plight didn't inspire any sympathy within me due to how unkind most of the characters are towards anyone not like them. That said YMMV.

On a personal note I thought the story in Oblivion was much better and the characters more likable and easier to root for. It's a shame that the most interesting characters in this game are the ones written about in some of many books you'll find scattered about the landscape.

All in all I'd say that while the main story is hit and miss in regards to quality there are plenty of great side-stories. It offers hundreds of hours of gameplay even without the addition of mods and the world it takes place in has a very 'lived in' feel thus adding to immersion. I'd give it a 9.5/10. Despite some of its flaws it's one of the best rpg's I've ever played.