So i just finished the book. I won't go to an in depth review, but I'll give my impressions.
All in all, it was a luke-warm book at best. There are times when it almost feels like the book WAS written by adams, but then suddenly Colfer's voice comes crashing through and you realize it isn't. After all the time I personally have invested in the original HHG books (they're my favorite books) it was refreshing to read something new involving what, at this point, feels like old friends. Zaphod and Ford didn't QUITE feel like themselves, but Arthur and Random feel well done to me. Trillian seemed sort of hit-n-miss. Some points in the plot (like the general absurdity of things that happen) felt in place for the HHG universe, but overall, the plot didn't seem like something you would expect it to go from the previous sets. As far as comedy goes, I didn't really find any parts that got more than a brief chuckle or head shake out of me, and nothing too terribly memorable. A large quantity of things from the previous books were reused and brought up quite a bit, which seemed out of place to me. For example, it seemed like the words "Froody dude" were in every other sentence from Zaphod and Ford, in extreme excess to what was in any of the previous books, which was one small niggle of mine that I really felt I just couldn't ignore as a whole.
At the end of the day, If you consider yourself a big HHG fan, i would recommend reading it, but be sure to set your sights down a notch or three.