Tolkien never came close to finishing it. What he had at his death was a vast body of writings on various pieces of Middle Earth history, philology, mythology and geography. Being written at different points in time from 1917 up to 1973, much of it was contradictory at various points.
It was published posthumously by his son, Christopher, who had the unenviable task of trying to weave a coherent narrative out of the most publishable and less inconsistent texts. That's not to knock what Christopher Tolkien did. I doubt anyone else could have done half as well in compiling the uncompilable.
The result was not what Tolkien had in mind for the work in published form. For one thing, his envisioned work would have been about four times as long.
As published, The Silmarillion is probably closest to a mythology.
I think that the best approach to Tolkien is to take it in the order his works were in fact published. To read The Hobbit first, especially as a child. And then to go to The Lord of the Rings. These are Tolkien's most accessible works. The history underlying each which Tolkien gives us glimpses of what makes for a richer experience.
For those who crave more there are the appendices at the end of ROTK. That gives a good deal of historical overview of the background of Middle Earth. Once you've finished that you're at the end of published works as Tolkien authorized them.
The Silmarillion, Unfinished Tales, and the twelve volumes of the history of Middle Earth edited by Christopher Tolkien represent in turn the next steps for the Tolkien acolyte. Each step represents less accessible and more incomplete, fragmentary or inconsistent work.
I'd never recommend The Silmarillion to someone seeking their first introduction to Tolkien. Neither would (were he alive) Tolkien himself.
And this means what, exactly, for the upcoming discussions?