I've read the original Dune series, the preludes and Butlerian Jihad.
Dune, of course, is brilliant. The rest of the original declines slowly to Chapter House, which I gave up reading, because it seemed to be nothing else but an endless circle of clever "wisdom" wrought into a storyline that hardly ever reaches some point of originality; like Heretics, its predecessor. I think the last two books are a boring and unworthy conclusion to what what started with Dune.
And you soon grow tired of Duncan Idaho.
But as for the Preludes.
I liked Atreides at first. The story of Pardot Kynes was rather cute, but that's really it. The books are well written, of course, but for me to like a book, it's gotta have something special. The preludes are completely devoid of originality. Sure, there are interesting things, but unlike in Dune, they are presented in such an open and boring way that the Preludes ruin a lot of the magic in the original. I believe it was a personal mistake to read the Preludes. I will not recommend them.
Ather thing is what also appears in Jihad: The obsession of sex and violence. Feel free to call me weak-hearted, but I think a writer should limit himself to how many intimate details he exposes to the reader. At first I was rather disgusted at what I believed to be a deviation from the original, but then I read Heretics, and found that Sonny Herbert is just following in his father's footsteps. Atreides wasn't too bad, but Harkonnen, Corrino and Jihad suffer with this problem. If you really feel you need to expose your readers to it this way, there should be a well-founded reason for it. That reason is, as far as I can see, non-existent in the Preludes.
*Spoilers!*
I'm also rather annoyed at the simplicity of which politics in the Preludes are described. Countless times Shaddam, the supposed Emperor, makes astronomical blunders, and Fenring's got to save him. The heighliner/no-ship incident seemed to solve itself too easily.
And finally: Where did House Vernius go? IIRC, Rhombur got his planet back in Corrino. But in the original series, the rulers of Ix were always known as Ixians. This means they can hardly be Vernius (Atreides allies) or the Tleilaxu/Sardaukar occupants. Besides, in Dune, Paul had this vision with ships from all Noble Houses surrounding Arrakis in the final chapters. Did that mean Vernius too?
There seems to be other inconsistences too; Duncan remembered his parents tortured in the Barony in Heretics. But in Atreides, they were, IIRC, merely prisoners, killed painlessly by Rabban.
In conclusion: The Preludes take away too much of the magic in Dune. Characters like Kynes, the Baron, Halleck, Yueh and (especially) Fenring seemed mystical; with the thorough explanations from the Preludes, they now seem reminiscent to celebrities who you suddenly find you know more than you really appreciate about.