It's rather different from LOTR. It expounds the creation mythologhy of Tolkien's world (quite beautiful, actually). The core of it has to do with a legend long before the world of LOTR. The narrative is not quite as vivid because unlike LOTR, the Silmarillian covers a much broader time span.
It makes the trilogy even more epic, IMHO, since it gives you a sense of the sheer ancientness of Tolkiens' world at the time LOTR begins. Like LOTR, it's a really wonderful "lose-yourself" read with a cigar and a cognac. :)
But since I am planning on rereading Hobbit followed by the trilogy in what order would you place it? First or last?