Agree with Dave...
I have a trusty old O2 XDA2.
I have a wired GPS receiver hidden behind the dash of my A3 that provides power and "normal" gps signals to my TomTom Navidator PDA software when I'm driving my own car.
I also have a bluetooth GPS receiver that I take use when I'm driving hire cars and also when I'm using the GPS as a passenger in someone else's car.
That way, I can even use it abroad - it guided me around California and around the streets of San Francisco last summer - from my hotel to Lombard Street, on to the Coit Tower and then to the Bay Bridge....
On top of that, I also have "Memory Map" software loaded with the 1:25000 Ordinance Survey maps of the UK on my 2gb memory card and use it for the odd ramble or country walk.
Its also a VGA camera and a video camera with sound.
Oh and its also my bluetooth mobile phone, linked to a bluetooth car kit and with voice dialling software installed.
I can get onto the internet at any open wi-fi point and can download my emails - and it also synchronises to my Microsoft Office works mail system to hold my calendar and contacts.
It might look a bit big as a phone - but its just one device that does everything.
When I had a separate phone and PDA, I still used to carry both devices around all the time because even if I wasn't using the GPS to follow a route, I use the GPS as a matter of course, most of the time I'm driving, to warn me of speed cameras in case I lose concentration....
I'm sold on the PDA solution and struggle to see what the built-in option gives you apart from something to talk about when you carry a new passenger.....
I too first considered GPS when I was buying a new car back in 2002 as a £1200 option and if I'd spent that then, it would have gone with the car when I sold that last year.
The £400 I spent on the PDA solution (2002 prices - a lot less now...) has provided a lasting and flexible solution that I wouldn't have had with a built-in fixed system.
Oh and I forgot to mention - its an MP3 and video player too, because its Windows based... You can get a 2gb card for just £15 these days.... Enough for several DVDs or loads of songs....