Let's get one thing out of the way: the game is short. Depending on the speed at which you read and the time it takes you to make decissions, you'll be through in a couple of hours.

That said, this is possibly the best introduction into the Word of Darkness, and more specifically the Werewolf: The Apocalypse setting you could hope for. Usually, in stories with a hidden supernatural world, the newly introduced character enters the world and then has to learn about it. Heart of the Forest though truly explores the introduction into the supernatural world. It is not a matter of you went out, you got bit, you now are a vampire or werewolf, here is their world. In Heart of the Forest you follow your character's path as she gets unwittingly drawn into this world. Obviously, you know what she doesn't, what she will discover, but this discovery is a gradual process until - no spoilers.

Rage. Every World of Darkness game (Vampire, Werewolf, Mage, Wraith, Changeling, Hunter, Mummy, Daemon (did I miss one?)) explores horror and human nature in a different way, but while players enter the supernatural world and drift farther and farther from humanity, this tie to humanity is what creates the horror, the dilemas, the drama, and what sets it appart from other RPG settings. Heart of Darkness handles Rage rather well. It isn't this inovative super clever mechanic - it is simply well done and well woven into the storytelling. As your rage grows you feel your character lose control. You see things getting out of hand, the trainwreck about to happen. If you know the setting, you may very well be feeling the the constant underlying threat of violence juxtaposed to the beautiful forest.
It is one of the things the game does so well, capture in its writting the atmosphere the setting, not simply interesting characters.