The book written as an appendix in the Guide to the entry on the Hitchiker's Guide is a welcome addition to the trilogy (of five) even if it cannot be the same if the same author would have finished the series.
Eoin Colfer in my opinion has understood well the dynamics of the story and has come up with a plausible continuation and introducing some interesting new themes.
And Another Thing has a similar kind of language to the H2G2, yet without such memorable phrases as "mindbogglingly", "almost, but not quite, entirely unlike tea", and "The ships hung in the sky in much the same way that bricks don't."
There are, however, many excerpts from the Guide to provide background on some of the developments of the earlier books (which are quite unnecessary for the real Hitchiker's fans) and for comic relief. These replicate the style of Douglas Adams but are a little too numerous and a tad bit weak in some cases.
Still, I would recommend this book as a fun and light read and a nice way to honour the work of a great author.