Much to my surprise - and just scratching the surface with seven RPG/Quest mods selected out of literally hundreds - the answer is "yes".   With just the mods I discuss below, I estimate you could easily get two or three times the hours out of Oblivion than by just playing the vanilla game.
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Vilja

It is easy to forget that the primary goal of an RPG is to draw you into the story so that you are invested in it - you feel a part of it.  To an RPGer, your imagination to picture what is going on still is far more important than thumb-speed to whack baddies faster.  If you look at those "best RPG lists" that show up so often, you might notice one common denominator for them.  All the best ones involve the player, capture you, and make it easy to picture the environment or quest in a way that makes you a part of the story.

A huge part of this potential "immersion" is the depth of the characters that are part of the story.  Even years later there are NPCs that we remember - such as Kelghar, Vi Domina, Minsk, Viconia, Neeshka, and (perhaps, in time) Morrigan.  Sadly, most others have faded away and are long forgotten.

And in the hundreds of characters I have enjoyed over decades of RPGs, Vilja is the first and only character that somehow became "real" - a "person" you can picture that becomes an integral part of the story.  I can think of no higher praise an RPGer can give a character mod than to say it became as real as a game environment can allow.  And it is for that reason that the Vilja mod for Oblivion is the best character mod I have ever enjoyed. 

Technically, Vilja is "just" a companion.  But that is sort of like saying a phoenix spouting fire and swooping down on a battlefield is "just" a bird.  What separates Vilja from any other companion I have ever played - mod or part of an original RPG - is her fully-developed "personality."
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