Being a bit of a gadgety person, Ive currently got
Garmin i3 Standalone
Garmin Quest Standalone
TOMTOM ONE - Standalone
TOMTOM5 - running on SPV C500 smartphone with BT GPS
TOMTOM5 - running on a DELL X50V with a BT GPS
For convenience, and reliability, the TOMTOM ONE is by far the best option just for SAT-NAV.. it just works brilliantly, it is faster then my Dell X50V (624Mhz), and the SirfIII built in chipset gets a lock very easily behind my heat reflective windscreen..
The PDA/Smartphone is fine, until you start installing 'other' software on the things, and some software conflicts (several Today Screen Plugins)..
Then you have to remember to keep it charged, and the BT GPS, which means two leads in the car, so the idea of a 'wireless' BT isn't quite so 'wireless' at times..
If you want a PDA for PDA things, then you might as well get a PDA with TOMTOM5, and just be careful when installing other applications.. It is less reliable, but then a quick soft-reset usually sorts it out.. and TOMTOM5 is more reliable for me then any previous version..
Now I have the TOMTOM ONE and Garmin i3, I have to admit, I don't use the PDA anymore for SATNAV, they are just that degree more convenient..