As a TomTom One XL owner I was extremely dubious about how well a multi-function device like a smartphone could touch a dedicated device, especially one which really does just work so well.
Initially I tested Google Nav in the car on my route home from work, seeing how accurately it mapped the route out to the sticks where I live. Just before I arrived home it said my house was so many yards up on the left hand side, a little detail which really impressed me. Then, when I arrived outside my front door it showed me a photograph of my house!. Fantastic!...or so I thought...
I was so impressed with the dummy run I thought I'd use it on a trip to Bristol Airport. I'd flown from Bristol a load of times, taking the same route, going through the city. It had been some time since I'd done it though so I thought I'll use my Desire and Google Nav as it had done so well on its initial test. What a bad idea.... it was harder to read in the more complicated city environment and I missed turnings... then the correction / redirection took too long, and then it seemed to send me round in circles!. I nearly missed my flight to Berlin, which was extremely stressful. It also seemed to interfere with my ability to make a bluetooth call to the airport through my car stereo. With all the fiddling about with that and the stereo it was just luck I didn't crash the car.
Never again!!... I'll only ever use it as a backup now, say if I forget my TomTom or it goes FUBAR en route.
BUT... I got to Berlin ok, and used the Google Nav steet mappiing to guide me round on foot - and it was a-mazing!. I'd been to Berlin a bunch of times so was reasonably familiar with the centre, but walking round some new areas at random was a breeze with the Google walking mode. Just brilliant!. Battery life was a bit of a concern with the GPS enabled so I was switching things on and off a lot to conserve power. That wasn't so hot.
So, the short answer IMHO is that dedicated devices are usually always better, unless of course you make pick up a dud in the first place.
Oh, and the free Berlin underground map for android was a real lifesaver too!. A highly recommended download if you are ever planning to go to Berlin :-D
HTH