I have an android phone. Google nav is better than iguidance hands down (and you can download offline apps anyway if gnav didn't cache far enough beforehand, which has never actually happened to me). Media management with android is only getting better. proper use of widgets and a custom launcher with a good frontend could one day make this a total carpc replacement
problem is, a couple things: a) screen size--if I want a larger screen I need to use a tablet, and right now android tablets just suck and don't have many connections for peripherals. and afaik I can't just video-out my evo to a 9" touchscreen and be able to control that instead of the phone
b) UI. this is the biggest issue--there's simply no comprehensive frontend for android. however, when development eventually takes off, the touch-oriented nature of android and its toolkit will make programming an attractive, functional frontend easier than it is on windows (although hopefully if win7 tablets take off new tools will be available for windows too
c) obviously nothing can replace the carpc for people who use functions like playing video games in the backseat, backup cams, etc., for the next couple years at least.
that said, I REALLY wish MS would release a version of bing maps navigation for windows, which i suspect it eventually will when win7 tablets come out. I've used an HD2 and bing maps is essentially on par with google nav. ones more touch-friendly development libraries become available for windows we'll see much more elegant carpc interfaces