Does anyone know some good ModelAdmin documentation? With some example will be better.
I found those following pages:
http://open.silverstripe.org/wiki/development/modeladmin
http://www.silverstripe.org/data-model-questions/show/5704

One more question. the ModelAdmin seems to be build around the dataobject, ie.
class Category extends DataObject
class Product extends DataObject

If so, does that mean that product can not be a page type to take advantage of the full html text editor. I know that in silverstripe popup form, one can only use a cut down version of html text editor - SimpleHTMLEditorField. Is this still true?

How about this...

http://www.silverstripe.org/making-a-crm-with-modeladmin-in-silverstripe-230/
http://doc.silverstripe.org/sapphire/en/reference/modeladmin
http://doc.silverstripe.org/old/recipes:cheat_sheet#modeladmin

If so, does that mean that product can not be a page type to take advantage of the full html text editor. I know that in silverstripe popup form, one can only use a cut down version of html text editor - SimpleHTMLEditorField. Is this still true?

I don't exactly follow this - but yes the popups don't have tinymce HTMLText, but you don't need to use the popup with ModelAdmin - it scaffolds the HTMLField as tinyMCE fine

(btw @Willr - these links should be sticky in this section I think, 3rd time this week I've mentioned them)