I finally watched it yesterday. Great show... but I am still a little in the dark of how it all fits together. Looks like people here disagree. All in all my two cents would be that the story is made up of three historical timelines:
1. the flashbacks - what happened before the crash
2. After the crash - they all died and had to learn how to let go - the island
3. After letting go - they find new ways to meet up and move into eternity together (the flash forwards of series 6)
And then all that gets mingled a lot. The confusing bits are the nuke, the Dharma people and Widmore & Co... If the island is indeed purgatory, damn, what a violent place it is...
Anyway, I still have some figuring out to do here...