I'm using a Vodafone V1615 (rebadged HTC TyTn II) which is currently discontinued, but it's basically a Windows Mobile device with both touch screen and a slide out QWERTY keyboard.
It run Outlook Mobile / Pocket Outlook which is a cut down version of the Desktop Outlook. However, for me it works a treat and syncs to the Desktop with no problems.
I'm also using 2 third party programs that greatly help with GTD.
I've TodayAgenda (http://tiews.info/index.php?show=sof...agenda&lang=en) which runs on the Today Screen and shows my calendar and dated tasks for upto the next 45 days.
It's always there on the main screen so I've no excuse for missing anything. Switch on the device and it's there, don't even need to go into calendar or tasks lists to see what's Today and the next few days too.
I'm also using DeveloperOne AgendaOne for the Tasks manager alone; this allows me to view by Context/Category/Project etc. It's also cool as I've setup several task templates based on context etc.
I'm also using the voice recorder, camera, excel mobile and word mobile for lists, quick capturing and project notes etc. I've also remapped many of the hardware key to open tasks, voice recorder, notes etc with a single press.
I'm due an upgrade this month but it's working so cool now that I've finally set it up just as I've wanted, I dont want to upgrade and have to set it all backup on a newer device!!
Let me know if you'd like any further info and screenshots etc.