Mighty cool to have GPS built-in to the phone. And this is the first time I'm owning a smartphone with that stuff. I've used Google Maps with my HTC Desire several times now to take me to places I'm not familiar with and 99% of the times it worked. Basically, it does what it should. However, very unfortunate that in Malaysia, there is no turn-by-turn voice navigation so I would have to often look at the screen to follow through.
The 1% that didn't work out was when I reached a road closed down for maintenance. I was redirected by signage to a different route, which at night, looked rather new so it did not show up on Google Maps. I was basically floating on empty land on-screen.
One other disadvantage I find is that Google Maps rely on constant connection to the web. I tried turning off 3G (but keeping GPS turned on) for just a few seconds and my position immediately floated off to some where else! I couldn't imagine using this to go off to some isolated place with no mobile internet signal or with the slow GPRS variant... not very reliable if out of the city, I suppose!