A good question to ask yourself when reviewing a work of literary theory is: what do you want the work to do? With much French theory in particular, the reader begins with very particular questions (whether they be personal or dissertation-related) that the book may or may not be interested in answering. Barthes--and S/Z in particular--is a case in point.
For anyone interested in unconvential techniques of writing or reading, the introduction can function as something of a manifesto. Much more radical than Barthes's more structuralist works, I found it immensely helpful in formulating theories of writing not situated in the individual subject.
The "critical reading" of the Zola story that makes up the remainder of S/Z, however, not only is intolerably boring, but appears to be a structuralist codification of the introduction's radicalism. Like much literary theory aimed at production rather than analysis (I'm thinking in particular of L=A=N=G=U=A=G=E poetics), the produced work almost always disappoints the model. In light of this, the question that arises is whether the work reflects the true intentions of the model or whether the work itself is merely deficient? For Barthes this is the eminent question. And in S/Z you can see him straddling his stucturalist and extra-structuralist tendencies with wildly contradictory results. For those not familiar with or interested in Barthes the thinker, however, the contradiction is bound to repel.
That said, S/Z is required reading for anyone studying literary theory or French intellectual history. If, however, your interest is more casual, check out Mythologies, a Lover's Discourse, or Camera Lucida.