And Another Thing makes a nicer end for Hitchhiker's than Mostly Harmless did, which, as I gather, was the point of having it written. Colfer's writing doesn't read like Adams's which is good because Colfer's own style is reasonably pleasant and I've read enough things by people pretending (unsuccessfully) to be Douglas Adams to last me Wowbagger's lifetime and bad because the reason I picked up the book was that I wanted to read a new Douglas Adams novel (alas, a thing that is not to be). Colfer pulls together a surprising number of unresolved plotlines from Adams's five novels and resolves them; he leaves enough dangling, however, to close with a meditation on endings (there aren't any, really). Arthur and company's ultimate fates are left up in the air, but this is much happier than the definite doom Adams had closed Mostly Harmless with. 
 So: very much worth reading if you care about the Hitchhiker's story and characters; unlikely to fill the bill if you're expecting Colfer to channel a new Adams tome from beyond the grave.