Awesome potential. My big frustration: There does not seem to be any obvious way to install as sidebar widget, although the plugin appears to insist that this can be done "in seconds." For the life of me, I can't see how it's done at all. Instead, contextly sticks a gigantic gray box at the bottom of posts. The functionality is kind of cool but the aesthetics leave something to be desired.
I'm certain the intention is for maximum flexibility but, at the moment, something big and obvious is missing. What that is, I have no idea. Hope the contextly guys figure it out soon, though!