This book starts at a disadvantage: I don't think Douglas Adams should have written a sixth volume, and I certainly don't think anyone else should, even with the encouragement of his widow and daughter.
Eoin Colfer is a successful author of children's books, and this reads like a young adult pastiche of Adams: the Bantally tree can perform hexes when it isn't hibernating, which sounds more like something from Harry Potter. Various things happen, but I'm not sure I'd describe it as a plot.
Colfer regurgitates most of the familiar characters (except Marvin) and situations in a repetitive, inconsistent and often abrupt and banal way. The excerpts from the Guide are sometimes too long and there is a rogue section that suddenly has about a page each of the inner thoughts of the main protagonists. In particular, he takes the delightful word "froody", that Adams used so sparingly, and peppers the entire book with it (three times in the first two chapters alone).
Although Adams had a bit of a thing about Thor and, as an atheist, enjoyed mocking religion, I think it is too major a theme in this book: Hillman Hunter (named after a car, like Ford Prefect) interviews gods to maintain cosmic order on his planet. Cue lots of silliness.
The only dash of originality is a nice Vogon who dislikes paperwork and killing.
However, I like the ideas of me-vangelists and symmetrophobia (a feature of a hideous Vogon ship) and there are glimpses of Adamsesque ideas and language: "Ford nodded with a wisdom beyond his ears"; "Zaphod stepped into as foul a den of broken dreams as he had ever been thrown out of and felt instantly at home"; "Fate was dropping him a wink, destiny was slipping him a brown bag, providence was beating him over the head with the hint hammer", and "handed... a gift-wrapped basket of mill grist".
Nevertheless, they don't salvage it for me. This book is anything but froody.