I love Gmail and anything that can boost it or make it better than it already is, is always welcome, especially when it comes to organizational features.
GTDInbox (formerly known as GTDGmail) attempts to "transform Gmail into a powerhouse of productivity and manageability". The aim to to give you a faster, more task-orientated inbox. When you install GTDInbox, you're presented with the setup screen as soon as you restart Firefox and open Gmail. It's apparent though from the beginning that it's not as simple as you might hope.
You have to allow GTDInbox to assign labels such as "S/Action" "S/Finished" so that you can label tasks with them. The thing is, you can make these labels yourself very easily in Gmail so I'm not sure what the attraction is of it. You are supposed to be able to do your most common functions within one or two intuitive clicks using a new popup command box that lets you preview, archive, delete or modify labels on your emails. I simply could not work out how on earth to use this properly and in the end, found it much easier to revert to the tried and trusted basic Gmail interface that I know and love.
You can however assign e-mails by priority, preview messages before you send them and multi-task by opening several windows at once with different e-mails in progress on them.
Overall though, I found GTDInbox far less intuitive than it promised and it should make it much clearer how to use it. Perhaps a video tutorial would be a good idea.