I think it's widely accepted that Dune Messiah isn't the sequel most wanted, and I concur. Dune was split into 3 parts chronicling the rise of Paul Atreides and this novel is really Part 4, rather than its own self-contained story. From that perspective, Dune Messiah is the victory lap that wraps up many of the themes that Dune set into action, and on those merits, the novel is a success and probably deserving of 4 stars.

But... Frank Herbert released this as a standalone novel, and it does not stand alone. The novel is half the length but the cast of characters is nearly as large as Dune, which means that many characters are nothing more than names. Some should be considered critical antagonists and yet they get one or two scenes at most. The arc of the novel is stunted as well, since the conspiracy that faces Paul has none of the clever foreshadowing and plots-within-plots that marked the glorious Harkonnen vs Atreides feud. Instead, it unfolds with us as bystanders, rather than secret conspirators the way we were told of Yueh's treachery, and the novel suffers for it.

Dune Messiah is required reading for the Dune series, but unfortunately, it'll feel like a speed bump compared to its predecessor. Do your best to read this immediately following the conclusion of Dune, and you'll be much happier for it.

Honestly I think the biggest weakness of this novel was focusing so much on Duncan Idaho, given the low page count, when he was given comparatively little screen time in the previous novel. Frank Herbert should have spent as much time fleshing out the conspirators as he did on Duncan and the genetic memories of the Gholas. I know in the future books these are critical events, but this novel was about Paul vs the Face Dancer, and that got maybe 20% of the focus.