Not sure what system you are considering, personally I have a
Ipaq 2210 running tomtom2 software with a bluetooth GPS reciever, to plan a route all i have to do is type in the city, street and street No and the Ipaq does the rest.
No planning on the PC neccessary, you can also add 20 favourite places and press navigate to ,home or uncle bob etc.
Another useful extra (downloadable from the link I gave you) is checkpoint where you can add POI's (places of interest) including a speed camera database which notifies you when approaching a speed camera location.
If you join the forum, there are some really knowaglable guys on the forum that will help better than I can (still a newbie to sat nav)