Essentially use Multiple Inboxes as well as some filters/labels to create the basic GTD categories. I've modified it to use Labels since I find the various stars inefficient. I have inboxes for "Work On" (label:Priority), "Someday" (label:Someday), "Waiting" (label:*Waiting), and "Reference" (is:starred). This combined with services offered from Boomerang (http://www.boomeranggmail.com/) get pretty close to what I was trying to accomplish with Mailbox. It's not as elegant but I don't have to worry about using a proprietary email client that might not be around.