I`ve been playing around with it since the release, and I find it useful so far (especially in terms of link-building). As far as I understood - it is based on all incoming links (internal+external) to a page, so it shows the estimate popularity and importance of the page from that point of view (and judging by SpyGlass guys have a pretty fast and powerful own crawler). Now it`s only the links, but I assume LinkAssistant is going to add a more complex metric in future based on that new InLink rank (like Moz`s PA/DA based on Moz Rank) so that the quality/risk factors would also count.
And how do you like it so far?