of course it was never built for car usage but it is a good fit out of the box. i am using it for weeks now. so i can judge
1. press and hold on power button for suspend. elsewhere not needed now
2. have not used them/i am not used to them
3. Archos has its own multimedia framework and plays almost all the vid formats/codecs
4. you define the style with the widgets you are choosing. with a nice background it is great. what i am missing is an app to periodically change background and/or visualizations in music to have a change from time to time
Check out http://code.google.com/p/android-obd-reader/, it should work on the Archos 5 but I haven't tested it. It would be awesome if you could test it and let me know! It is also in the Android Market.