Yes, it is.
It satisfied the purist in me because it's pulled almost straight from the Arcueid route. But it changes enough things (Akiha vs Ciel) and fills in enough holes (such as explaining Ciel's past) that it actually winds up being better than the source material. It also has the advantage of being able to show you how much ass Nanaya kicks instead of letting you read about it.
I still urge people to pick up the game, but I'll be damned if the manga isn't fucking awesome.