I've used a Garmin c320 in Thailand, and based on this single-device experience, here are my thoughts :
1. The interface is very important, as you don't want to be distracted by multi-level menus, etc while driving. Garmin's was pretty good, with simple menus and clear diagrams. Ability to program waypoints would have been nice, but I guess you gotta spend a bit more, and have it panel-mounted as in an Airbus
... you can sort of fudge it with the current interface, but it's not that intuitive.
2. Signal acquisition ... obviously you'd lose signals inside buildings and under flyovers, but sometimes multiple high buildings will block the signals as well. The Garmin was ok in this respect, it lost signals but reacquired pretty quickly.
3. Positioning ... I'd see a PDA-type system having a disadvantage here, it's nice to have it on the periphery of your line of sight, ie. suctioned on the front windscreen, so you don't have to really take your eyes off the road when you're checking. Contrast this to something mounted to, say, your dashboard. Having said that, you'd probably have to mount the PDA to the windscreen anyway for the antenna to work.
4. Software ... the software in the Garmin by default took me to the shortest route most of the time no matter what I programmed. And in Bangkok, at times it took me to the most congested routes ... on one trip I took 30 minutes to get somewhere (as I was familiar with that direction and went on the expressways), but going back took me 2 hours (could not get back on the expressway, plus jam).
I'd see it of limited use in Singapore, but usage in other countries will be good. It recalculates routes for you if you miss turns, etc, and given time, will get you to your destination no matter what.
Another bit of bother is that you need to bring it with you when you leave the car, otherwise it's perhaps a bit too much of a temptation to sticky-fingered louts.
HTH