I have a 32GB 4th gen iPod Touch and I use it as a mobile personal computer. I used the description PDA (personal digital assistant) for a short while when I purchased my 8GB 3rd gen. As the useful apps began stacking up it was clear to me that the iPod Touch was far more useful than any PDA I'd used up to that point. I get data service anywhere through my phone which functions as a WiFi hotspot so it's like having a mini iPhone without the voice radio.
I use my iTouch for the usual PDA stuff: calendar, tasks, notes, contacts, email, and documents. All of this syncs online, most of it to Google, as do my andrid phone & tablet and my netbook & desktop PC's, so that all share the same information locally. I also use it for browsing the net, research, e-book reading, as a media player (photo, video, and music), encrypted personal data storage, calculator, unit conversion, currency exchange rates, travel checklists, check flight times & status, shopping online, making reservations online (food, travel, etc.), IP subnet calculator, basic network tools, reference for just about everything (home & appliance repair, electronics, math, spelling, grammar, thesaurus, dictionary, medical and medication, etc.), facility maps (airports, malls), and a myriad of other useful applications. It's definitely my most used personal computer. I find new things to use it for all the time it seems. Because of its excellent battery life and its very small profile its the ideal device for an every day, full- time mobile personal computer.
I also have a HTC Sensation 4G android phone. I find it every bit as useful as the iTouch with some major hardware and software advantages. I use the Sensation for some specific things: GPS, mapping, navigation, traffic, weather, still & video camera, photo editing, FM radio, complex document editing on the go, video player, voice calls, video calls / chat, and as a WiFi hotspot for my Android tablet, my iTouch, my wifes iPad 2, and my boys iTouch when the power goes out or we're on the road. The phone uses up its battery faster than the iTouch and so I use it mostly for voice calls, camera, and GPS/ mapping / traffic, and try to use the itouch for most everything else.