Google Nav and CoPilot
I've tried both GoogleNav and CoPilot on my Streak (Android 1.6) recently, and both seem to have the same problem of not handling phone calls well. I've only tested this whilst stationary, but both have the SatNav screen completely replaced by the incoming phone call, and after the call (using bluetooth headset), it returns to the home screen rather than navigation. This isn't exactly ideal, and if I was the sort of person that got lots of phonecalls, then it would render the mobile close to useless for this purpose.
What I'd expect, is a subtle on-screen indication of a phone call which doesn't block the navigation view.
My other problem with CoPilot is that I can't find a way of displaying estimated journey time (apparently it's possible, but I can't find a way of making it work and anyway, it should be there by default). Most of the time, I know roughly where I'm going and use the SatNav for deciding whether I have time to stop off at a service station before my meeting. Not having this feature makes me want to stick with Google Nav, despite it's lack of other features.
The 5" Streak however is definitely big enough to use as a satnav.
One last thing - the option to switch between overhead and 3D view is really nice (my old SatNav, a PSP, has it). It switches to a large scale (configurable) 2D view if the distance to the next event is greater than 3km. Generally, if all I'm doing is travelling along a road, being able to see how far along it I am is far more useful than seeing the turnoffs I'm going to be ignoring. Especially useful on the M1, where I can be on the same road for 3 hours+.