People just like to hate popular things. I also read Dan Brown's novels in high school/late middle school and remember thinking they were exciting page turners. They were never as good as most of Michael Crichton's novels, but on par or better than most of Cussler's IMO.
Maybe the prose really was repetitive and the plot banal, but it took genuine skill to write something that you physically struggle to put down.
I agree; I read them a little later in life than you and GP did (early/mid 20s), but I enjoyed them just the same. "Page-turner" is a good descriptor. I found the stories themselves to be fun and engaging, even if the writing and plots weren't the best.
Maybe the prose really was repetitive and the plot banal, but it took genuine skill to write something that you physically struggle to put down.