For one thing, it's absolutely dangerous. There are tons of buttons, and pretty much all the car computers use some kind of selection wheel and context-sensitive buttons. That means you have to look at the screen to know what mode it's in to know where you should scroll and what the button will do. Nothing in a car should ever require you to take your eyes off the road. I know you get to memorize the menus a bit over time, but every car I saw the menus are very deep and complex, and people are inevitable seduced to go fiddle into their car status and check their average speed and tire pressures or go edit their address book during traffic, and that means unsafe driving. I think most people now finally would admit that talking on a cell phone in a car (even hands free) is very dangerous (though it doesn't seem to stop anyone from doing it) - the car computer is really no better.
It's a terrible trend that so many cars now have the radio and air conditioning and such settings buried in these computers, which means just for normal adjustments that you want to make while driving you have to look away from the road and fiddle around.
But say you just don't give a fuck about hurting other people or crashing yourself. I still think car computers are retarded. One problem is just that they are generally rubbish. Okay, let's pretend that's not a problem and your car actually has a really good one. You're still faced with what I believe is the most fundamental problem :
Car time scales are not the same as electronics time scales.
That is, in 5 years your car might still be very usable and great, but in 5 years your computer will be shit. Computers should not be tied to the car. In 5 years your nav system will be laughably poor compared to cutting edge, the display will suck, etc. etc. Even if you like car computers, they should be a separate pluggable component, not built in. Furthermore, car development times are generally long, so by the time the car comes out, the computers in it are generally already out of date.
Hell cars can be kept 10-20 years, and in those cases the computer becomes a really annoying anachronism.