Whoah whoah whoah. Lets not get carried away. He clearly states in the video that they are using an Audison sound system in that vehicle, with two amplifiers. A quick look on Amazon shows me the -cheapest- Audison amp is ~$250, going all the way up to > $1000. That's JUST the amps as well. Add in additional hardware, an iPad, and not even counting installation (if you could do it yourself lets say), and you are probably over 2 grand for what's in that car.
I'm not saying its not cool, but there is NO WAY you would beat the quality of a "$1000 stereo" with an iPad for less money. The iPad is just putting out a stereo line level signal, the rest is all being done by the expensive audio hardware...
I'm not an audiophile, but I wouldn't be surprised if one could tell the difference in audio quality compared to a device more specifically geared towards putting out high fidelity sound than an iPad, which is intended for use with -at best- some dock speakers.
Personally, I wouldn't want a setup like this, because in order to use your car's stereo you would have to have the iPad in place, and as mentioned by others if you don't remove it when you leave the car it begs to be stolen. The end result is you now have to always carry the thing with you (or sometimes not use your stereo). Maybe that works for some people (cool!), but would not for me. I personally would find it a burden to have to keep track of one more device at all times.
All just my $0.02 of course.