Well, in all I guess it depends on what the in-car use of the tablet would be and how much you value functionality and features over usablity (ie UI). For my purposes I'm mainly interested in music (both local and internet-radio) and gps fuctionality. I mean these are the functions I that I use on a daily basis. There are loads of apps in the Android market that meets those needs (for me). I can put up with the lack of a nice in car gui and be on my merry way (Implicity, this is what most of the people with iPad installs have decided). Like most people, I prefer a hassle free way of doing simple things that are important to the functionality I need.
For example, I currenly use Centrafuse's webradio (internet radio) pluggin. When I want to listen to specific radio streams, I have to find the mms link for the station (can be a pain to find) and update an xml file and hope it plays nice. In contrast, I use Tune-In radio app (on iOS and Android) for the same functionality without the hassle of worrying about links etc cuz all the stations I need are preloaded and managed by that vendor. The interface is not something I'd really worry about right now as it's doable. 1. How important is it if a developer merges this interface into a car friendly app for me? Not very. 2. How important is it that I can just press play without mucking around? Very.
I know, my views might not be shared by all, but I'm one of those people that like to know that can easily add functionality by going into an app store and looking for what I need. There are thousands of Android apps, while not perfectly scalable to a tablet today (this will be addressed shortly by future releases), still provide additional functionality to the car in a hassle free way.
To be honest if Apple released a double din friendly tablet (which I know from Job's last earning call, they never will) I'd probably be looking at that option instead.