I only watched the 1st and half of the 2nd series of Lost. I stopped watching it after reading in Sky Magazine an interview with the show's creators, in which they stated that they didn't know how many series there were going to be, but estimated that it would be 4 or 5. This indicated to me that they knew how the show would ultimately end, but were obviously planning to 'kick the arse' out of it until it's popularity (and profitability) started to wane. At only 3 or 4 episodes into series 2, I already thought it was running out of steam/ideas and couldn't bear the thought of sitting through another 2 series.
I remember commenting to friends/relatives at the time and since, that it would more than likely end with everyone finding out it was a dream, similar to the Prisoner. Or, would end with everyone finding out they were dead (a la Sixth Sense) and the Island was some sort of Heaven/Afterlife.
So, after waking from a short Baileys induced Coma on my sofa at 5:20am, I was flicking through the channels and came across the Lost finale. Out of interest I decided to watch and I'm happy to say that I was nearly right and I'm so glad I didn't waste my time watching all 6 series.