I just have the default Google maps function on my iphone 3G, and it is pretty much worthless for navigating while driving. If I had the Navigon software (or whatever it is called) it would be much better, but would still need a better GPS chip it seems, and also a window mount.

My car, which I bought new in 2004, came with built in GPS, and I use it all the time. The software is dated though relative to a modern nuvi. I also have a nuvi 260, but don't use it much - have built-in and don't want someone breaking my window to get it. Otherwise I would have it on the window all the time - prefer it over the built in, except for the smaller screen.