I use my iPod Touch as a PDA much more than a media player.
Google has a new beta application called Google Calendar Sync that syncs outlook and Google Calendar very well (only one calendar, though) so syncing my iPod Touch to Outlook now keeps it in sync with Google Calendar.
As for mail, I use Gmail exclusively, so it's kind of a toss-up for me between the Mail client and the iPod Touch-friendly Gmail Web site. Which one I use really depends on my circumstances. If I'm online and I want to read and manage my email, I prefer the Web version. But if I know that I won't be near a WiFi connection for a while, I just do a quick update to the Mail client when I'm online, and I can read and respond to email while offline using the Mail client. They are both useful. (I just wish the Mail client would sync with Gmail directly via WiFi.)
Also, though I have since Jailbroken my IPT, I paid for the January Update specifically to get the Notes app. (yes, I felt that simple application was worth the $20$). This elegant yet simple application lets me jot notes quickly and easily anywhere. My only complaint is that it doesn't sync--not with Outlook or plain .txt files--so there is no easy PC-based data entry.
ToDo list apps are simply not available unless you Jailbreak. And the couple available are nice but very rudimentary. And they don't sync or integrate with Calendar. Lately, I find myself just keeping a ToDo list in the Notes app.
A "stock" iPod Touch makes a nice, but not very sophisticated PDA that has completely replaced my Palm PDAs. After owning numerous Palm PDAs since the original Pilot 1000, I have to say that the iPod Touch really is what a Palm should be by now. I hope that the SDK will produce sufficient PDA-oriented applications to make it even better.
(And to be completely redundant to many of my other posts, my main application is Wiki2Touch which has transformed my IPT into an amazing, efficient information resource. Yes, Wikipedia anywhere, any time. So cool!)