I can't recommend these books enough. After the ending of Frank Herbert's Chapter House and his death, I really wanted the world to pass a law that if you are thinking about dying you can't end your novel in a cliffhanger.
These two books are a work of love that ties old and new Dune together in a complete tapestry that makes the 20 year wait and all the prequels worth every moment.
If you like Dune, you need to allow yourself to be amazed at how respectfully and faithfully Dune has been expanded and concluded.