In short, THE SILMARILLION is the Bible of Middle-Earth. Epic battles, the struggles of certain chosen individuals, the creation of the world and more, all told in a beautiful "and so it was" sort of tone. True to form, even the names are difficult to remember (thank goodness for the "Index of Names": I read almost half the book before I discovered this handy tool tucked away in the back of the book)--think Finwe, Fingon, Finrod, Fingolfin, Finduilin and, my personal favorite, Finarfin.
But if I sound flippant, forgive me, because I mean absolutely no disservice to THE SILMARILLION. It's a doozie of a summer read, sure, and if you haven't read The Lord of the Rings AND The Hobbit several times, you probably won't care enough to make it past the first hundred pages--but if you loved Lord of the Rings enough to read it twice then please, read THE SILMARILLION, because it is an absolute work of art and it fills in all the gaps that The Lord of the Rings doesn't have time to touch.
Beginning with the very beginning of Middle-Earth, and ending approximately one second before The Lord of the Rings picks up, THE SILMARILLION covers a lot of ground and an awful lot of history. I was surprised by how vast the book is, and I loved reading the stories of Earendil and of Numenor and the Grey Havens, just as I loved learning more about Galadriel, Elrond and Sauron (who are, surprisingly, just about the only characters you'd recognize for LOTR who are old enough to appear in THE SILMARILLION). The book is all the backstory you could possibly want, and I tip my hat to Mr. Tolkien and his amazing imagination.
And about his imagination: one of the things I loved best about THE SILMARILLION was the way the history of Middle-earth ties in with the lore of our own, by way of stories that connect loosely with the myths we tell ("Atalante" seems a bit reminscent of "Atlantis", especially given the context). This connection shows up in the way that The Lord of the Rings leaves off (welcome to the Age of Men, and all that), but is even more apparent in THE SILMARILLION--I don't dare give anything away, but you'll see.
So, any of you who love The Lord of the Rings enough to read the whole series multiple times, who own all three extended versions of the movie and have considered naming your first born child either Pippin or Galadriel (depending, of course, on the gender of the kid), get out there and read THE SILMARILLION right now. If you haven't read the book, liked the first movie, but was too sad about Gandalf to watch the second one, don't bother. You'll be bored out of your mind.