With characteristic good timing, I found myself suddenly interested in delving deeper into Tolkien's mythology... right around finals week. (Naturally, this led me to lie awake at night thinking about biological transport phenomena in the Two Trees of Valinor and so on.) But now I have a lot of free time and very little internet access, so I'm working my way through the first volume, The Book of Lost Tales I.
It takes a lot of getting used to. Everything's name is different, and the tales are interspersed with commentary from Christopher Tolkien, which is very insightful but detailed to the point of neuroticism. There are a number of subtle differences between this old material and the published Silmarillion, and very often I'm not sure which version I like better. (For example, in the Lost Tales, Ossë is a much more ambiguous character. He plays an important part in the story of the Lonely Isle, rather than just being "the Maia in charge of waves and storms".)
However, the big difference is that the whole history is framed as a series of tales told to a human mariner, Eriol, who comes to the Lonely Isle and hangs out with the Elves. And, well, it really doesn't work. Neither Eriol nor any of the tale-tellers is developed at all, although we are promised that actual plot events will happen later -- but they really need to happen closer to the beginning. The effect is basically "heroic adventurer arrives in distant mystical land and gets the Bible read to him for several weeks". It doesn't stand well on its own, but only in relation to LOTR, Hobbit, Silmarillion, etc. Let's just say I'm glad the stories were extensively revised before being published.
However, I'm only about halfway through the first history, and there are twelve, if I remember right. So I bet it gets interesting later. And after that, there's the Letters, where Tolkien gives a bunch more explicit commentary on the more philosophical ideas of his mythos -- mortality as Gift of Ilúvatar in particular. I'm looking forward to it!