as an anarchist with strong individualist tendencies (as well as a deep-rooted and irreconcilable difference of opinion with the many self-identified anarcho-capitalists i have talked with and read), here's my take. as always this is a very personal perspective; rice boy did a decent job of answering in a less subjective manner.
i am an anarchist (with strong post-left leanings) who simply refuses to place any group or collective (society, commune, church group, @collective, whatever) above myself. i prefer and choose to relate - as an individual - only to other individuals. that is to say, i choose not to identify myself or others as part of a group, whenever i can avoid doing so (which is most of the time). combining that with the core tenets of my anarchist perspective (anti-authoritarian - in which i include all institutions such as the state, economy, religion, science, etc - voluntary association, mutual aid, diy, etc) results in what some would call individualist anarchy.
anarcho-capitalists may well have individualistic tendencies, but they also retain ideas inherent to capitalism which i reject strongly. those have been addressed many times here on this site. most prevalent in my mind is the inability to see the world in anything other than economic terms, with every kind of relationship being reduced to economic transactions. most communists i have known have a similar tendency towards an economic perspective, if perhaps not quite as dogmatically so.
i guess to summarize, my version of individualist anarchy is rooted in anarchy, while anarcho-capitalism is rooted in capitalism - which i find to be undeniably at odds with anarchy.