First off: I really enjoy playing this game. I'm currently 70 hours in, and have only just passed the Cahbaet sewers part. Being the explorer that I am, I love going into every single location and looking for every single bit of treasure that can be found; I'm trying to get a full set of every type of armour. Nehrim, as opposed to Oblivion, really brings out the treasure hunter in me, it offering the unique armour and weapons that it does! Also: the game is extremely atmospheric; I love the game world - it is very real and immersive. As previous comments pointed out; the underworld was an extremely fun ride, and the scale of it very impressive.
Of course, you have to consider that the Nehrim developers didn't have to develop everything from scratch and that they borrowed all/most objects, textures etc. from Oblivion. Regardless, I think Nehrim did a better job at creating a believable world, storyline and fun experience. What I would like to see in Nehrim though, would be the possibility to buy a house in all of the major cities and would've liked to not have seen the faces of the developers in game (on some posters here and there). Those are just small things though, and I would love to see more coming from SureAI! Some major lag here and there made the game a bit unplayable, at times, unfortunately.
Everyone who knows about this game and has other friends who enjoy RPG's (and have played Oblivion) should make them aware of this great mod!