I use a BT GPS receiver that came with TomTom for the Palm TX. It uses the Sirf 3 chip which has a quick lock-on time.
My car has an athermic glass windscreen which blocks the GPS signals quite badly so I put the BT unit on the rear shelf of the car. When I'm walking in a city, I put the BT unit in a pocket on my back pack. This solves the lock on time problem.
On the other hand, I find once the internal 810 GPS chip has locked on, it hold on just as well as the BT unit.
What I like about the external unit is that you can leave it locked on but just fire up Wayfinder when you need it.