Ditto although unless its changed you need a data connection for Google Nav so get a contract with cheap/free internet.I'd second an Android smartphone. Has Google Navigation (iPhone doesn't). Just buy a mount from eBay and off you go. Talks to you, tracks you via GPS, gives you options for routes and shows live traffic. And it's free.
I have an old Tom Tom 500 which I don't use since it fell off the windscreen breaking the sucker off the bracket- £20 for a new bracket!
Got the Tom Tom app on my old iPhone which has a bit of a delay on the map refresh due to poor GPS signal but works OK... though I have to keep it plugged into the car's USB to keep it charged up.
I have an old Tom Tom 500 which I don't use since it fell off the windscreen breaking the sucker off the bracket- £20 for a new bracket!
If you're thinkng about the smartphone option and do much driving abroad you may also like to consider the price of data roaming, because google maps download on demand instead of being stored on the device
I have velcro tape - about 1.20 from Lidl.. then can swap the sat nav between my wifes and my car..sorted.. those sucker things aren't the best. a bit of tape on dash and on windscreen.. the soft side on the edges of the garmin.. never moves.
I reckon dedicated PNDs (Personal Navigation Devices) are going the way of the dodo, it just doesn't make sense to produce a one trick pony like that.
I have a Galaxy Tab which runs many different Sat Nav software solutions, and the big screen is a definite advantage (and yes, mine is mounted in a way that doesn't obscure the windscreen. When I get to my destination, it becomes a regular tablet computer.
As other have said, pick a solution that stores maps locally and doesn't require a data connection.