Sartre's existentialism is based on a misreading of Heidegger. Sartre is a humanist; Heidegger an anti-humanist.
In Being and Time Heidegger outlines what he calls the existential structure of Dasein (human being). This underlying structure allows the human subject to show up as the thing it is in occupying a public station of being (father, mother, son, worker, runner, teacher etc). Dasein's understands its current modality as its existentiell mode of being. The existential scaffold is what makes the existentiell subject possible. Because Dasein is essentially "interpretation all the way down," its existentiell roles do not ground its essence but are that which it presses into in order to flee confronting its own death (what Heidegger calls its ownmost potentiality). This potentiality is a groundless nothingness in which Dasein is not able to be.
Sartre misunderstands this in equating the existential with the existentiell. He gives a subjective reading of Dasein, whereby he illegitimately equates "consciousness" with Dasein. The Heideggerian structure of Dasein extends to the world in which it finds itself such that we cannot pluck Dasein out of its world, or in any sense consider it an isolated subject in a world but must always acknowledge that it is already bound up with a world. The existential structure of Dasein is that which founds a clearing in which "subject" and "object" can show up. Sartre's subjective reading completely ignores this fundamental ontology of Being and Time that establishes Dasein as being-in-the-world. Sartre introduces a crude ontological dualism whereby Dasein is for-itself and non-conscious things are "in-itself."
Whereas Heidegger's ontology has gained acceptance even in some "analytic" circles, with his attack on the Cartesian subject proving extremely useful in A.I., Sartre is widely regarded as a weak philosopher and he is more regarded for his novels than his philosophical works. However, despite his misunderstandings, I do believe he has some powerful insight into inter-personal relations, particularly in the context of master/slave relationships such as all types of sadism and surveillance. However, I'm afraid he is not really an essential thinker.
In a certain sense, Heidegger provides a non-theistic model for "post-nihilism" to set against Kierkegaard's existential faith. For Keirkegaard, the only hope for a meaningful life that could survive catastrophic misfortune and illness was belief in a Christian God. Sure, one could be an athlete or a hedonist, but one could break his leg or lose his sight through illness and hence be deprived of those two avenues of meaning, leaving one to despair in nothingness. Only a faith in God would survive the worst that might befall us. For Heidegger, even though the core of Dasein was nothingness, and the anxiety of this nihilism bound up with the very structure of Dasein itself, Dasein could become resolute via a call to embrace this nothingness authentically. Resolute Dasein understands the nothingness of its ownmost potential for being (that it is temporal and ultimately being-towards-death) but projects itself back into the ways and roles ("for-the-sake-of-whichs") of its culture. In this way Dasein dwells within its culture and does not simply flee its ownmost truth through diversion in the "they-self" (or "one-self") of those who seek escapism from the burden of their mortality.