I have been using Tracks for almost a year now. I have tried every GTD app on the Mac and online, and I really prefer online for a couple of reasons:
1) There are about 5 GTD apps for the Mac, but they are very new (and buggy) and diverse and don't quite match the simplicity that the GTD system intended.
2) If you are without your Mac somewhere, why not be able to get to your GTD wherever you are? No reason to tie your life and GTD organization to one machine. And once you move into the online GTD world, you will find tons of different implementations to match your needs. Some of my favorites are gtdtracks.com, taskfreak, and tiddlyspot. You can find a bigger list on
GTDTools
Remember, keep it simple, that's why I like hosted Tracks at gtdtracks.com. If it isn't simple, then it isn't something that you are going to use frequently, which defeats the purpose of GTD. So watch out for those eye-catching implementations and feature-filled packages, just stick with the basics.