I'm not a huge Hitchhiker's fan, but I still thoroughly enjoyed Douglas Adams' characteristically witty, sardonic prose. Though the game has a (deserved) reputation for being difficult and at-times cruel in its design, the world-class satirical writing and absurdly fun sci-fi narrative are more than enough to motivate the player to meet these challenges.
The first half or so of the game largely follows the plot of the first Hitchhiker's novel, and occasionally draws verbatim from the novel, though with enough wrinkles, puzzles, and knowing tricks thrown in to make it more than a straight adaptation. A working knowledge of the novel will certainly help a player of the game, though having read the book does not at all make this first portion of the game redundant. Adams (apparently a huge fan of IF) and Infocom veteran Steve Meretzky build in lots of charming, self-aware details like warning players to don 'peril-sensitive sunglasses' before viewing a low score after they've died, or a death sequence where the story continues following the deceased Arthur Dent in the ambulance and scolding the player to stay out of it. There's just a lot of learning by death in this game, but it's usually fun if infuriating!
The second part of the game diverges quite a bit in its design, moving from the more or less linear plot following that of the novel to a non-linear episodic design where the player departs from a central map to points across time and space. I really, really liked this except for some major details regarding the mechanics of this episodic structure that are not divulged to the player. It takes quite a bit of finagling (or consulting a walkthrough) to understand how to (Spoiler - click to show)handle the Infinity Drive and get in and out of the darkness, none of which is explained to the player. For many of the episodes, it's also not very clear what the objective is or what the player needs to do to make sure they don't end up in an unwinnable state. Nor is it clear how many episodes the player needs to go through before advancing to the concluding sequence of the game.
In full disclosure, I made heavy use of a walkthrough to get over these difficulties. Even still, I greatly enjoyed this game, and found its design inventive and its writing winning. I feel like a bit more direction about how to navigate through the episodes in the second part of the game would have added to the player experience without necessarily sacrificing any difficulty of the game. As it is, it feels like parts of the game design are cruel just for the sake of being cruel. Regardless, I highly recommend playing this -- and no shame for using a walkthrough at some key points.