We're very much pushing folks to use ad_form (which calls the form-builder) to implement self-submit forms in order to get the in-form error messages. ad_form's still not generalized to the point where it covers all cases that people want to implement but it's getting closer.
That will be our preferred way to build forms "by hand." To get a very simplistic overview in a real package checkout the "populate" package in the contrib part of the OpenACS 4.6.1 tarball.
And of course the track being discussed here is to move forward another step, automatic form generation and handling where possible. I'm not going to butt into that discussion at the moment (haven't played enough with the existing CMS form stuff to feel comfortable talking about its pluses/minuses), just wanted to emphasize that ad_form, with its ad_page_contract declarative flavor, is what we want people to use if possible rather than call the form builder "by hand". It's easier and more readable when the form handling you're implementing fits what it can do and it's being extended on a regular basis to cover cases it can't currently.
