A sixth book in the Hitchhiker's Guide To The Galaxy series came out last year, by an Eoin Colfer.
THHGTTG, any book by any author which attempts to continue it, will be met with a storm of criticism. Heck, even if Adams secretly had written it himself and people thought it was somebody else, many would hate it from the word go. But in any case, I really enjoyed it. I thought it was inventive and funny. Okay, there were a few details in the middle I thought had escaped the editor's knife by mistake, but not a lot.
So in other words, don't buy or read it if you feel strongly about Douglas Adams, or feel that, for example, a Zaphod Beeblebrox who has had his left-brained head taken off to run the spaceship is sacrilege.
And of course you may also want to steer clear if you think that the idea of another author continuing a seminal work is simply Wrong. I'm not sure, I can see both sides of that one, very often it feels like it seriously dilutes the originals. For example the Asterix books have been weak since Goscinny died and Uderzo took over the writing on top of doing the art. Ah well.