Personally I've gone the PDA/Tomtom route, mainly because I already had the PDA, so just needed the software, GPS reciever & mounting kit (you can either get the expensive tomtom kit which includes all the above, or save some money & get the bits seperatly). Cost me about £200.
Advantages:
Cheap, if you've already got the PDA, PDA can add £2-400 to the cost above.
Can be moved between cars
Can plan routes when away from the car
Will intergrate with a speed camera database & alerting system
for free
PDA still works as a PDA, and if you have the addresses in the right format can use your adress book to select destination
You can have several different maps, and buy more when you need them
easily upgradable
Disadvantages:
Lots of unsightly wires, unless your good at routing behind the trim
Uses up cigeret lighter socket
Depending on car mount, may not be in best position for visability
Volume for spoken directions is not the best/
Directions will not overide radio/cd - so not for those who like their music loud
