I using a HTC TyTn II (re-badged as Vodafone V1615) running Windows Mobile 6.1.
Using the inbuilt Tasks and Calendar apps, synced to Outlook 2003 on my desktop PC.
Not found any need yet to run 3rd party PIM apps, want to keep it as quick and as simple as possible.
Got the usual @contexts setup in tasks and can quickly view them with a single key press.
Also use an 8gb MicroSD in it, that with a mini USB card reader easily fits into any PC for syncing/transferring captured notes and files etc.
Created lots of shortcuts for quick capture and viewing action lists.
Key apps for me are:
Claunch 12.04 - create custom shortcut on the Today Screen. I've several groups of shortcuts such as capture, action lists/review, email/internet.
TodayAgenda - show upcoming calendar and tasks on the Today Screen. This free software alone has meant I've not had to purchase any third party PIM software - highly recommended.
AE Button Plus - brilliant for remapping keys for shortcuts etc. Map the same key with short press, long press, double press even triple press! This way a single key can open task list, open Word document "Inbox.doc" for quick capture, create a new next action or a new text message etc.
Word Mobile (built in) I've a word document called Inbox.doc that is great for quick capture; it's mapped to a single key and is stored on the memory card. Opens in a second, capture the idea/task, close and it's captured. On the desktop a similar shortcut opens it from the memory card too for processing etc.
Excel Mobile (built in) - spreadsheet setup to track spending and bills etc to tally later with accounts via online banking.
Think that's about it!
Cheers,
Andy D.