I've got an X50V + Tomtom5, SPV-C500 with Tomtom 5 Mobile and a Tomtom ONE (I bought this for the missus)..
If you primarily want GPS, then I'd go with the standalone TOMTOM ONE. PDA's work well, but like PC's, they can be screwed up quite easily if you are not careful requiring re-installing etc..
I more and more use the ONE as you just turn it on and it always works.. no resetting (For me at least!), it comes with the car mount etc, and just works.. very very convenient, I always use this on private journeys, however I do still use my smartphone for any work trips, as I always have this on me (Well I did until I upgraded to a Nokia N80 and I'm still waiting for TOMTOM to support it)
That's not to say the PDA isn't good, when it's all set up it works fine, but it is more expensive (TOMTOM ONE = £230, PDA + GPS + TOMTOM5 = ~£400)