I have been using (with some success) Outlook for my GTD implementation and and I am currently trying David's Add-In.

I have also been "shopping" to see if there is a better solution, including considering creating my own with MS Access.

Whether you use Outlook or any other PC / Web based tool for GTD depends entirely on your own circumstances.

For me, I use a desktop PC at my office 90% of the time. My company uses Outlook and Exchange as standards for email and Outlook therefore is my largest collection point, and the vast majority of my "Next Actions" stem from emails I receive. So, Outlook is a natural for my GTD implementation.

It's not perfect. My single biggest beef is that the task manager does not have the ability to track "Projects" that have Tasks and related sub-tasks.
I deal with those outside of Outlook.

My second biggest beef with Outlook is Printing my Next Actions Lists. (I want more -easier- options for formatting).

As far David's Add-In Goes, I'm leaning away from it. It has a few nice things - A new "Field" called [Project] for categorizing tasks; an easy to use way to turn an email into a task and delegate it in one step; and a few others. However, the Add-In is not needed for most of this.

You can simply drag an email to the Tasks Folder to make it a task. I like the Assign Task feature that is in Outlook a little better than the Delegate feature in the Add-In because you get the option to have the person you assigned the task to to email you Status updates on the task easily.

Take a look at
1. "Take Back Your Life!" by Sally McGhee from Microsoft Press, or
2. "Total Workday Control Using Microsoft Outlook: The Eight Best Practices of Task and E-Mail Management" by Michael Linenberger

For tips on setting up Outlook for GTD.

Note: Sally McGhee has her own system not affiliated with GTD but is so similar you won't care. Apparently Sally and David worked together at one point.

- My 2 cents worth -

Appelq