I cannot get custom fields to be added to posts or users at all.
After a bit of analysis, I can see that the $slt_custom_fields['boxes'] array is correctly populated within the slt_cf_register_box() function.
When the slt_cf_display_post() function is later called by wordpress, the $slt_custom_fields['boxes'] array contains zero elements. It's being reset somehow.
This is on WordPress 3.4.1