I first read the Silmarillion over a decade ago, and just finished it for the second time. Unlike The Hobbit and The Lord of the Rings (which are set in the Third Age), The Silmarillion is mostly set in the First Age, and is more about elves than men. The version I had also includes (as well as the precursor bits) the Akabelleth [a 30 page or so summary of the Second Age] and a similarly concise retelling of the Rings of Power.
It’s the highest of high fantasy, telling the creation of the world from before its birth, through to its early days when the gods were fiddling with it by themselves, and then (the lion’s share of the text) telling the tale of the elves. To be honest, I like it a lot more than The Lord of the Rings. Although the span of time it covers is enormous, there isn’t much wasted space. Instead of endless detail, time is devoted to interesting escapades (Beren’s adventures, for example) without the excessive padding that, for me at least, makes The Lord of the Rings a little too fat.
The Silmarillion is a great book of world-building (in both the literal and story-telling sense), covering Arda from before its creation to the final events that are described in greater detail in The Lord of the Rings. A potential downside is that, after the initial part, it can be damned tricky remembering just who certain elves (and, later, men) are, and how they’re related to one another. Elves being immortal makes this more difficult than it would otherwise be.
Because the author doesn’t dwell needlessly on less interesting events, the pace is good despite the enormous scale of time involved. It is not essential reading for The Lord of the Rings, or The Hobbit, but it does help fill in some background knowledge and is interesting in its own right.
I’d strongly advocate checking a sample, however. The writing is substantially different to other works, and I suspect some people would loathe it.