Wow, folks are actually talking about one of my favorite books! Of course, the reasons I found it fascinating are the same reasons people cite for its lack of appeal.
I actually like the Biblical structure and voice. It makes it seem more like a dry, fragmented history of ancient, half-legendary events, rather than a story.
However, Tolkien’s The Lord of the Rings isn’t for everyone, and The Silmarillion is much, much less so. What we’re looking at is essentially Tolkien’s background notes for an entire world in which he was going to set his main stories. That probably puts it one step above the entertainment level of getting a script from a movie you like.
So, I wouldn’t recommend the book to anyone who hadn’t read or didn’t really like LOTR, and even then I wouldn’t expect them to like it. On the other hand, I still get geeked by knowing that Sauron wasn’t the first evil in the world, or even close to the most impressive. :)
(It seems to me that The Silmarillion performs the particular function, however, of making Middle Earth seem more real. Not because of the background materials, but because the myths and stories follow the familiar pattern of “Everything was bigger, grander, and more dangerous back then” that seems to be a human storytelling trait.)