I admit, I was biased against this book from the first. Anybody but the late and much lamented Douglas Adams trying to write in the Hitchhiker's universe is basically on a hiding to nothing as far as I'm concerned. There may be somebody out there who could write a H2G2 book and make me think that Adams had risen from the grave, but Colfer certainly isn't the person in question.
One of the problem with his effort is that he tries too hard really - he throws in references to the other books at madcap pace as if trying to distract the reader from the fact that he isn't Adams while actually reminding them of it at every turn. And he will insist on using "froody" as an adjective, when, if I recall correctly, Adams only ever used "frood" as a noun. It's a tiny thing, but it's like a tiny bit of gravel in your shoe - it begins as a minor irritation but eventually becomes very painful.
In fairness, it's not an awful book - there are occasional funny bits and some of the plot is quite well constructed and there are probably a great many authors who would have done an even worse job. But it's not Adams and it was never going to be.