In the preface to "For Marx" Althusser states that he wrote the work in order to reinvigorate the ailing tradition of marxist criticism. I believe that it is in this light that one should understand "For Marx" and its highly controversial interpretation of Marxism as an anti-humanist and structuralist science, for by adopting this approach, Althusser negates the tradition of for example the young Georg Lukacs that emphasises the humanism/Hegelian idealism in Marx.
The success of Althusser's bid to revive Marx by, so to speak, setting up an antithesis to the prevalent view of Marxism can be seen in the influence which he has exerted over a number of (post)modern marxists critics, notably Fredric Jameson. Indeed I believe that there exists a high degree of applicability of such Althusserian concepts as "overdetermination" and "ídeology/science" to the postmodern ideological praxis.
On the negative side, I must admit that Althusser's interpretations and methods often seem artificial and somehow constructed ad hoc, and his anti-humanism also seems excessive.
Nonetheless, this does not detract from my general impression of "For Marx" as an interesting and relevant work that has not deserved the neglect which it has endured since the seventies. In sum, I am for Althusser.