Awesome.
A lot of franchises have attempted to make expanded-universe stuff that ties in with the plot, and it's pretty forced, but this way was done very naturally...
It kind of glossed over the backstories of major characters in the game, but also fleshed them out a great deal...while I liked the Ratman's little intermittent dens of sanctity, he also kind of creeped me out the way he obsessed over the Companion Cube and his paranoia of GlaDOS's ever watchful gaze...
This helps put in context seeing as how he was there the day she turned on them all, and the way the little Cube just glows in a warm little pink light and hovers over his back, chiming in helpful little bits of advice is just charming.
Glad the Cube finally became a character (with dialog and motives of its own) in its own right.