I'll give Herbert credit for writing a second Dune book. I thoroughly enjoyed the original, and so many people regard it as a masterpiece in the genre. It was an ambitious book that definitely deserved a better film adaptation (BTW, go watch Jodorowsky's Dune if you haven't already). Dune Messiah doesn't cover as much time as the first story, and to be honest it lacks something that made the first book so readable.
Much of Dune Messiah is spent with a host of supporting players plotting against Paul, and of Paul upset his mate can't conceive. Also, self-loathing. Paul's sister Alia gets much attention here, no doubt a big set-up for her role in Children of Dune, which is next on the TBR. You see potential for a strong female presence in a sci-fi, and in the end she's crying for a man. Eh.
As a Dune story, I liked it, just not as much as Dune.