i have been planning to do the car thing for some time and with this round of discounted e dots, i got an extra one to play around with. here are my observations:
-the echo does take a long time to start up but this could be a boon (see last item).
-the new ability to configure with one device only (hotspot) works for only one power-up, the next time you power up, it won't work anymore and you have to setup all over again. this has been mentioned in other threads and other sites.
-if you configure using two phones/devices (i used an ipad), you can configure the echo to connect to the phone hotspot just like any other wifi connection. the echo will reconnect to the hotspot again when it powers up.
-the problem i encountered was that when i started in the driveway, it connected to the wifi in the house. apparently the connections/passwords are stored at amazon for all your devices. no practical way to disable it unless you create a different amazon account(?). so actually if you drive away while the e dot is powering up, eventually it will connect to your hotspot. in my case, i just powered it off and on after i drove off.
-i am on a T-Mobile unlimited data plan. on their basic plan which allows lower res video, the e dot could not stream continuously. i had to go for their "plus" plan that allows for HD video for $10 more a month. still pretty cheap considering it's unlimited data.
for car use i think, amazon should improve the boot time and also have a way to set a "preferred" connection.