I've been working my way (sometimes back) through the entire, expanded Dune series. I started with the original Frank Herbert books, read several of those co-written by his son after Frank's death, and am now starting at the front of the story, working forward. This this trilogy, Sisters of Dune, Mentats of Dune, and Navigators of Dune, is the 2nd trilogy. They really can't be read or reviewed as 3 separate books. All of them (plus the 1st trilogy) are exceptionally dark and really highlight various broken aspects of human nature.
"Mentats" continues the back-story to Dune. In my opinion, it gets much darker than it needs to. The worst side of humanity seems to be the unifying factor in this trilogy. Again, it does not concentrate on the story of the Mentats, rather it jumps around through multiple agencies and institutions, setting up the ancient history for Dune itself.
I rated this a 4 because there are too many obvious turns and sub-plots. I found myself thinking "I know where this is going", and being right, far too often. The newer books also tend to develop a lot of clumsily concocted names for the actors. This trilogy also has a few inconsistencies that conflict with the original Dune story. That said, the imagery of the writing is beyond compare. The authors come very close to capturing the genius of Frank Herbert and create stories that grab your mind and take it into the future.
I've enjoyed the books and would strongly urge anyone reading the series to start at the story-line (not publish date) beginning and follow it from that perspective.