Most sci-fi adaptations fail to capture the underlying novel, so that's not a surprise. What I always thought
was the surprise was that Hubbard's book was so popular in the first place. Hubbard himself said, "I hope you enjoy this novel. It is the only one I ever wrote just to amuse myself," and I have always wondered whether there is any correlation between your intended audience and quality of the story.
Still, you asked about "decent",
@gabriel alexander, and by any measure,
Battlefield Earth was a roaring success. I'd love one of my novels to be a New York Times Best Seller and earn $1.5M in 1980s money! In that regard, it's like
Fifty Shades or
Jonathan Livingston Seagull for me: one of those poorly written books that catches the zeitgeist and makes a motza despite being poorly written.
As for Hubbard's other work, he did not write a lot but to be fair, some of it was well received, including
Fear, which Stephen King gave kudos to.
I think, as with any author/novel, beauty is in the eye of the beholder and some find Hubbard beautiful...and some don't!