I just can't see the same guy who played Anton Chigurh or Raoul Silva as a dignified warrior leader like Stilgar. I'd rather see Jason Mamoa (who will play Idaho) as Stilgar, actually. Oscar Isaac is too young to play Leto Atreides.On the other hand, David Bautista as Rabban, Stellan Skarsgard as Baron Harkonnen, and Charlotte Rampling as Reverend Mother Mohiam are absolutely spot on.
Here are some of the other announced actors:
Timothée Chalamet (who???): Paul Atreides
Rebecca Ferguson: Lady Jessica (any pretty face could play her)
Josh Brolin: Gurney Halleck (meh)
David Dastmalchian: Piter De Vries (this could be genius or horrible)