A few of us here at TUAW have been playing with the private alpha of Things for a while. Things provides a unique and flexible way to handle tasks that can be refreshing for those who didn't quite get the whole GTD thing the first time around. Of course, you can adhere to strict GTD principles with it, but you can also forge a system that works for you using its tagging system and flexible project management tools. It even has some delegation capabilities that other GTD apps are lacking right now. Plus, it's downright pretty.
What Things is noticeably lacking at this point is a really great means of integrating with Mail and AppleScript support. Of course, if there were AppleScript support, Mail support would be a cinch. You can pop up the quick entry window with a hotkey and drag a Mail 3.1 message onto the notes section to create a link to that email; I'd just like to be able to incorporate Mail-Act-On or any faster means of getting my email-based tasks into my task manager.