This is all spoilers, spoilers, spoilers, so it goes under the fold...
Most of all, it reminded me of Jacob's Ladder and especially Michael Powell's Stairway to Heaven (A Matter of Life and Death), two movies worth rewatching in any case. The final scene, while the credits roll, is simply that of a plane crash with no survivors. I view the show's cosmology as reflecting the existence of all possible universes and we get to see, and live with, a few of them. That includes the universe where they all die in the initial crash, the universe where they all die in the hydrogen bomb explosion, the universe where the hydrogen bomb creates an alternative reality, the universe where there really is a miraculously surviving "Oceanic Six," the universe where the main island narrative happens, the universe where it is all a dream of Jack's, and bits of others as well. This Leibnizian move "explains" the show's numerous unanswered questions, such as those about the lottery numbers and many more. It was possible, so it happened, toss in the anthropic principles as well.
The most striking moment of the final episode was when Locke tells Jack, quite sincerely, that he does not in fact have a son. The question remains how the different universes fit together or interact and in some manner it seems they do. The final episode is extremely effective in bringing out the dreamy and speculative tones of many of the previous episodes.
Most of all I viewed the ending as tragic. It was not mainly about any particular account of the metaphysics of the island. It was about how few couples had the chance to actually live together, love together, and stay together. The perfect reunions of the couples in the "we're all dead" scenario only drove this point home. I found this contrast moving.
At the end, the door is left open for Jack (the body of Jack?) to become the next smoke monster on the island and you can spot some clues to this effect, such as Jack's body being strewn on the stones in the same manner as it was for The Man in Black.
I saw two major weaknesses in the denouement. First, Widmore is dispatched too summarily in the penultimate episode. That thread of the story is not so much hanging (which would have been OK), but rendered irrelevant. Years of dramatic gravitas were swept away in a single, hastily executed murder scene. Second, Ben is a weak and poorly defined character in the final episode and runs around like a puppy dog, with no clear moral stance. Since he usually dominates any scene he is in, this is strikingly incongruous.
Overall I thought it was the best final episode of a series I have seen, with close competition from The Sopranos.