I didn't have a PDA personally, but people I knew did, and I'm pretty sure they did things like check the web, listen to music, take (low resolution) photos, an

I didn't have a PDA personally, but people I knew did, and I'm pretty sure they did things like check the web, listen to music, take (low resolution) photos, and check their email (which is what most at my workplace apparently use their smartphone for nearly exclusively). The other things you mention are a function of the improvement of the hardware or the existence of services (twitter, facebook), not being a completely different category. I think to get to a similar appflow to what a HUD enables, you'd have to have both the iWatch and voice commands, and then you'd still be stuck raising your wrist to read from it, meaning you can't operate it hands-free. HUDs combine all the best uses of iWatch and bluetooth mic/earphone combos, and improve on them.