I would put him well above Grisham personally. For my money, Koontz is one of the most under-appreciated writers living today. And I don't think whether a writers works have been translated into successful movies or not is even slightly meaningful as a metric.
For what it's worth though, I totally acknowledge that that is my subjective opinion.
“The Watch” is so far removed from the source material to the point where both Neil Gaiman and Rhianna Pratchett have denounced it as sharing no DNA with pTerry’s work (“Batman if he's now a news reporter in a yellow trenchcoat with a pet bat”), that I would be hesitant to even count it as an adaptation.
Beyond that, some that I would recommend include The Bad Place, The House of Thunder, The Door to December, and Intensity.
I cannot let go of "Lightning" from '88. That book has stuck with me in ways only a therapist could properly interpret, but activates all my historical fiction cilia and leaves me w/warm feels.