After 6 seasons, the ending was satisfying for me....My take on this was that their lives on the island were
actually lived...Jack's dad, Christian, confirmed that, at the end, when asked if it was all real. Otherwise, no one would have had any reason to want to spend 'eternity' together with people they simply shared a flight with.
The plane crash shown at the end had no tell tale markings, so it could have been the plane that had just left with some of the survivors on it, or even another plane, with other people. Also, Hurley thanked Ben for having been a great 'Number two', If they had not continued to be the 'guardians of the island' that exchange would make no sense. Altho many questions remained unanswered, that's ok with me, as the finale was so emotionally satisfying.