Robby's totally right with respect to Sartre. And a statement that doesn't hold for Sartre doesn't firmly stand as a statement about existentialism, I think; he's too central. Virtually everything I've read of Sartre's engenders a triumphant resolution to create the world I want to live in, and a sense that doing so is my personal responsibility. His philosophy is not all ponies and rainbows, because it's sometimes extremely frightening, but it's the opposite of depressing.