I actually watched the entire show and thought it was worthwhile. The season before the last is kind of strange and I am not really happy about how the end turned out as the last season has a "what if the plane hadn't crashed" story line that intersects.
But part of the construction of the show was that the audience actually played a role, if in a small way, with the narrative itself. This is in part seen by the alternative story lines, the meta narrative, that was happening off-show as watchers surfed the web looking for hints to the story.
Part of this meta-narrative was marketing, but also an appeal to fans to stay involved, and as a reward they would gain insights into the show.
A friend of mine actually kept a fairly developed blog on lost and actually wrote a Lost guidebook, that sold fairly well. We'd have these debates on a regular basis. He shared WesDude's view that there was an intended beginning and end, but that the story was broadly defined over the 5 or so seasons of the show, but that a lot of the details were left out.
In my opinion, Lost was a much better series than a lot that is on television now. It demanded a lot of viewers to keep up with it. But then, a lot of the better television does.
In many ways, television drama has become far superior than a lot of films largely because you get very good production value matched with the ability to tell more complex, detailed and character driven stories. Think of some of the best television dramas- The Wire, OZ, Deadwood, Sopranos, The Shield, and many others- these complex stories that are character driven but spread over multiple episodes. Even the Walking Dead has that formula- production value story but character driven.
Lost was great in production detail, but it excelled in the mix of story/character relations. Key too good story is character. I think Lost, by virtue of the multiple season, does that.