If you don't need to be led by the hand - err, I mean navigated - to a place without stopping and just want a way of locating an address/POI often with more accuracy/info than a map/Sat Nav then another option is to get a Smartphone with GPS (iPhone, Samsung Omnia etc..) and Google Maps (especially if you're due an upgrade and can get one for free).
You can always install SatNav software into the phone if you need to be led but to be honest Google Maps is so much better for visualizing because of the aerial view and now Streetview (Limited coverage but growing).
As a courier I use the Zumo but also have Google Maps on my Omnia and many times I've been saved by the search capabilities of Google when my Zumo hasn't recognised a postcode/address.
Of course a smartphone with Google Maps may not be as practical to view on the go as a SatNav (would need to be kept somewhere vibration free and dry!) and you would need to pay �30-35 a month on contract to get a decent phone package eg. free phone with unlimited internet & texts and 600 mins talktime.
(It does help, though, that all bike bits and accessories are tax deductable for me!!)

Cheers,
Rick