In my opinion, Dune Messiah isn't particularly great, at least compared to the superlative Dune. However, Children of Dune actually ties all three books together in a way that seems like it was designed as a multi-generational epic. I read God Emperor of Dune, but it takes place thousands of years after Children of Dune, so it really begins a new cycle of the tale. While interesting, I put it down after reading it and wasn't inspired to pick up the next book in the series. So I'd say read through Children of Dune and then stop. It's a perfectly good stopping place, and ties everything up admirably.