I have never enjoyed Lost so much since season 1.
Finally, in season 5 we are treated with revelations about the islands history that reveal a few surprises. The relation between Ben, Charles Widmore and the Island is revealed through the various flashes. Flashbacks and flashforwards are now used in a completely different way to reveal the struggle of our Losties who have changed in many ways since the first season. The mechanism in which this is done may confuse people, but just bare in mind that this show introduced an evil pillar of smoke in the first season, that sounds like a machine and blows up trees!
Top notch acting from all concerned but Michael Emerson (Ben) and Terry O'Quinn (Locke) in particular since they, in my opinion, portray the most interesting characters. I find Kate and Jack quite boring, although they are more interesting than usual this season.
The only gripe I have with this season, is that it did not reveal much (outside of character development) about a certain organisation that I wish to know more about.
The season finale of this season is the best of of them all so far. Finally, after the events that take place, we are given a real clue to what is really going on.
My warning to newbies to Lost, treat Lost like a book, a journey! Do not watch season 5 unless you have seen all previous seasons, it will ruin the experience!