>If the form is complex enough to use the form designer then using DO FORM simplifies the process - you don't need the extra step of Save as Class and avoid the possibility of having the class not being updated.

I believe there's no principal difference.
Form designer is so similar to class designer (I think it's the same code at all). So, you need to Save as Class once only if you decided to move to class designer. Then you can kill form .scx file and continue development in class designer. New form class can be started just as a subclass of the Form base class.

Main advantage of OOP approach to forms is subclassing. For example in my basic dialog class I have OK, Cancel, Apply and Reset to Default buttons with corresponding code. I don't need to add them and write their code again every time I create new dialog. If I need to change something in that generic code, I just need to edit parent class, not every subclass.

Only one advantage of DO FORM is that this way form can return value from Unload event. But there many ways to do similar thing with a form class.