They are rubbish. Aside from being poorly written -- plenty of groan inducing dialog -- they completely distort the historical details from the original books. Rather than being an extension of what is philosophical series, they are instead cheap space-opera and power-fantasy.
Sandworms of Dune was such a disappointment that I'll check the books out of the library now. The "prelude" and "legends" series were good enough that I went out and bought my own copies of them.