I use Outlook 2003 along with the GTD Addon on both my desktop and my laptop, I also use a Blackberry to keep on top of things while out and about.

Basically I do all of my processing on the laptop or desktop using the GTD addon, the Blackberry lets me filter out junk mail from my inbox and perform quick replies, it also lets me use tasks and it's filter function to drill down on my task list at the GTD action level. Perhaps the most common use I have for the BB is to filter the task list to @calls and then close them off one by one or to review information from my task list or notes while within a meeting.

If i do process my inbox on the BB, anything that can not be dealt with directly goes into a folder for immediate processing into a next action when I am back in front of a machine with Outlook installed.