Pros:
Unlike the black and white of the Hobbit or Lord of the Rings. The Silmarillion is quite different in its tone, it reveals a very dark twist on creatures such as elves, men and dwarves and brings us back to the time where the armies of Lord of the Rings were but tiny specs compared to the free folk or darkness at the time. It introduces us to the first great dark lord, Sauron's mentor, Morgoth. There are some legit sad stories in this massive novel of last stands and deaths.
Cons:
The book is huge. It's crammed in with so many different stories and reads Tolkien didn't fully finish expanding on. What's more, it's like reading a history book so you better be prepared to read some factual writting. In some places the book may also confuse you so you must read the Hobbit, LOTR, Unfinished tales, etc, before you start this novel.