I have them working very well, it took a good bit of debugging especially concerning custom fields in SFDC or Marketo. Due to a lack of documentation, I had to do a bit of debugging.
For the custom Gravity form fields you need to use the API name of the field, not the friendly label name. In the Marketo interface, if you go to Admin -> Field Management then click on "Export Field Names", you will get a xls file that contain the API name of all your Marketo fields. Look for the custom field you are having trouble with. Use that in the WordPress Marketo Feeds section of WP.
Worked like a charm for me.