"There was Eru, the One, who in Arda is called Ilúvatar; and he made first the Ainur, the Holy Ones, that were the offspring of his thought, and they were with him before aught else was made. And he spoke to them, propounding to them themes of music; and they sang before him, and he was glad."
Wow, Book of Genesis much? To be fair, the prose style and plot structure makes a certain amount of sense, because Tolkien was a devout catholic all of his life and his original intent for The Silmarillion when he started writing it in 1914, was to make a unified mythology for the British Isles. So it stands to reason that he would want to write the book in this way. And while I personally think this choice makes it feel more epic and poetically beautiful, to the casual reader it might come off as dry, highfalutin and very, very hard to get into. And if that wasn't bad enough, the book is infinitely more complex and dark than either of Tolkien's more famous work. So you really have to pay attention or you're gonna get lost very quickly. But that's not to say that The Silmarillion isn't worth your time, because it definitely is.
As far as plot goes, well that's kinda hard to explain. As I've said before, the book doesn't seem to have an overarching plot, it feels more like a collection of novellas that all focus on the same thing, namely telling the history of the first and second ages of Middle-Earth. Some of the stories presented here will sound familiar to Tolkien fans, as some of the characters in Lord of the Rings make references to the events depicted in The Silmarillion. So you'll read the stories of Beren and Lúthien, the story of the Silmarils (which Bilbo reference to in The Fellowship of the Ring), the rise and fall of Melkor (AKA Morgoth, the Dark Lord before Sauron) and The Fall of Gondolin. It also includes stories that, while no less important, fans had previously never heard like The Children of Hurin and The War of Wrath. The book also has the origin stories of many of the key players of Tolkien's other work like Elrond, Sauron etc. So in that sense, The Silmarillion lays down the foundation for the culture of Middle-Earth, as well as its early history. It's a fascinating read that can help better your understanding of the Middle-earth mythos, but as I said before, there are a lot of things to keep track of so its easy to get lost if you don't pay attention.
To sum things up; despite all of its flaws, The Silmarillion is still a must read for any Tolkien fan, or just a fan of the fantasy genre in general. It's a beautifully written swan song for the father of the fantasy genre. If you have the chance, then check it out ^_^. You'll be glad you did.