I just finished reading Flynn's introduction to Existentialism. Although he mainly focused on Sartre, he discussed Kierkegaard, Nietzsche, Heidegger, Camus, De Beauvoir, Marceau, Merleau-Ponty, and a few others. This is the second of the Cambridge "Very Short Introductions" I have read and they provide a very helpful and (so far as I can tell) representative outline of the subject matter.
What is interesting to me about Existentialism is its contradictory poles of direction. One important result of Existential thought is the pervasive nihilism resulting from the inevitability of a fractured existence. But, the existential response to nihilism is not abandonment to it, but something of an "embrace," which accept the inevitability of existence while at the same time asserting the conscious choosing or willing to be what one is determined to be. Kierkegaard's "single individual" and Nietzsche's "ubermench" are examples of the will to be what "I" choose to be in the face of all other inevitabilities. Kierkegaard in particular felt the inevitability of what he called "Divine Governance" and because of its overwhelming presence in his mind he forsook marriage to his beloved as well as a quiet life pursuing a small parish pastorate.
But where Kierkegaard would emphasize a "better" way, many existential thinkers simply argue for the "different" way. Nietzsche denied absolute morality and thought that the true individual was free to choose whatever morality that individual saw fitting in response to the loss of any absolute directive. One would suspect anarchy to be the common denominator of such thinking, but existential thinking would deny this conclusion, because whatever common morality it might recognize at all must include the maximum freedom of choosing for each individual. Oppressive moralities would supposedly contradict this ideal construction and thereby be invalidated by existential sentiment.
So it would seem, on the face of it (and indeed, I think, at the bottom as well), that existentialism does not escape the necessity of positing absolutes when pressed to choose for themselves what ideal they would advocate under their philosophical title. And if they would deny this title it would still seem inevitable for them to choose an absolute. But perhaps the inevitability of such choosing it part of the pervasive nihilism ascribed to existence and though we are all to blame for "absolutizing" whatever morality we choose in the choosing of it over and against another choice, it is the existentialist who can push through the anguish of contradiction and the angst of guilt in asserting oneself over the other because at least the existentialist can recognize the seriousness of the situation and feel sorry for it.
But if my characterization seems too much like caricature, I can soften the blow by saying that I have much more yet to read before I can conscientiously come to any conclusions, tentative or otherwise. I do tend to pick on philosophies of life that assert, indeed hail contradictions as paradox, as mystery, or as the inescapable reality of human thinking. What would deny the necessity of clarity on deep questions of meaning and what would impugn certitude as prideful arrogance, while remaining certain of certitude's uncertainty as well as the pride of those who assert certitude as a thing to be desired, what would lead its followers into faith in the freedom to choose with conviction from a cesspool of arbitrariness deserves a bit of cheerful retort once in awhile, don't you think?