if you are a beginner like myself, this book can be a good start. The book itself is actually a speech delivered in Paris in 1945. Its language is very clear and readable. The basic concepts of existentialism and the criticisms leveled at this doctrine are expressed in a lucid way.
The book consists of three sections. The first 20 page is introduction by Philip Mairet. The introduction provides readers with some backgrounds about existentialism, which i believe does a good job. The second section, from 23 to 70 pages, Sartre lays out the basic concepts of existentialism and differentiates atheist existentialism (advocated by Sartre, Heidegger etc) and religious existentialism ( advocated by Kierkegaard and Karl Jasper etc). In this section, Sartre also addresses to the major criticisms leveled at his philosophy. The final chapter, from 71 to 95 pages, is basically the Question&Answer section after the speech,some people ask questions to Sartre to clarify his position further. Apart from the last 15 pages, it is not difficult the follow the book. But in this section, the discussion between Sartre and a guy called M Naville is getting more and more abstract and make that part of the book hard to comprehend.
My overall impression is that it is a good beginning both to Sartre and Existentialism. Particularly when we consider how difficult the topic is, it is fair to say that Sartre did a good job in his plain style.