Tom,
I don't myself know much about either the form builder, and I've only used ad_form once. My experience with ad_form was positive and I found the resulting code was quite concise and readable.
As far as documentation goes it seems to me the proc is already far above OpenACS average. I would like to have some guidelines though about when to use ad_form versus the form builder directly.
I am curious to know what you have been trying to do with ad_form and how that failed.
When I find a proc that is missing documentation or has incorrect documentation I try to amend that. I encourage others to do the same.