"Mostly Harmless" was, IMO, not very good, and a rather lame ending to the whole Hitchhiker's series. Most of the jokes were not funny- the best line/joke is a scene with Ford Prefect at the Hitchhiker's offices. I only read it once, as opposed to the others in the series (which I've read several times each), so maybe it goes down better with repetition, but I've just never been interested in seeing for sure.
As for spoilers, I really don't recall enough of the story to give anything away (which, in and of itself might tell you something about the quality of the story- not very memorable). Something having to do with the end of the universe and the Hitchhiker's Guide being the key to its salvation or something. All in all, if you never read it, I don't think you'd be missing anything. I prefer to believe that the series ended with "So Long, And Thanks For All The Fish."