I found Dune Messiah to be more accessible than Dune, which isn't to say I didn't enjoy Dune. But it was dense, and felt sometimes a little too much like he was trying to emulate the faux-history style of Asimov's Foundation, but not quite as well.
Dune Messiah is more written as fiction, and so the story is easier to follow, the characters easier to fathom. And yet, while I read it quickly, it didn't have that deep of an impact on me. I'll probably read it again some day, but I wasn't really moved by the story Herbert was telling.
But as a follow-up to Dune, and an examination of the Empire started in the first book...well, at times, it glossed over things I either missed in Dune (entirely possible) or were never explained. I don't really feel like I "get" the religious war. I mean, what the religious principles they're fighting for are. And what they want to accomplish. Or how they're doing it. This feels like something I should know.
So, in summary, more accessible, yet still rather opaque.