"Lost" lost its plot line (such as it was) after season 2. When it became a hit and was renewed, the writers all said to themselves, "Jeeze, what do we do now?" So they all did some (more) coke and really started winging it. It's amazing to me that mish-mash still has viewers. But, as H.L. Mencken said: "Never overestimate the intelligence of the public."
I'm completely unspoiled, have no idea, and am loving it that way. I go back and forth on whether or not I think things might have reset and at the moment I'm on the side of "yes." But I don't think that means that the first 5 seasons are meaningless- if the reset happens, the characters will probably get back their memories so that even though things never happened physically they'll still have had an impact on the characters and the relationships formed can stay intact.
I agree with dabeaner that after season 1, the writers were clearly making it up as they went along. Many people (including me) stopped watching because they were annoyed by that. But giving the series an end date means that every episode has to advance the story (No more Nikki and Paulo!). With that said....
I think that everything will be reset, but they'll somehow still end up back on the island one by one, OR everything will be reset and something will happen that makes them choose to "undo" the reset, putting them back on the island.