Although I liked some of the features of GTD Inbox (it made the "switching labels" thing a lot easier for one) for some odd reason the combo of Inbox and FF3 made Gmail lose the ability to allow me to pick new colors for my labels. Since I'm super visual and love the colors, I uninstalled Inbox.
It's totally possible to have a GTD system in Gmail without the add-on. All you have to do is simply label your messages. I use the same old labels--@action, @waiting for, @maybe. I archive and label reference messages, and keep all status messages in my inbox--my real "inbox" in the GTD sense is the top of my screen where the most recent, untagged messages are. Some people archive everything, including their @action and @waiting for e-mails--it's a preference thing. If you archive, then you'll need to click on "@action" to see your actions, but it does give you that beautiful "inbox zero" feeling. One thing I like about keeping my e-mails in the inbox, though, is that after 50 messages I have to go to another page. And if I've got more than 50 actions and waiting fors, it's a signal for me that something is amiss with my system--either I'm procrastinating on some some short-but-longer than 2 min replies or my Waiting Fors need to become "@action--follow up"s. If you frequently have more than 50 e-mail NAs, then maybe you'll want to archive.
Oooh oooh! I forgot one other thing. In g-mail, you can create unlimited e-mail addresses that go to the same box by following your gmail handle with "+text." Then you can write rules about how to deal with e-mail coming to email@example.com as opposed to yourname @ gmail.com Send an e-mail to "firstname.lastname@example.org" and have gmail automatically label those e-mails @action. It's pretty.