I have one and it does the same as the lower wattage iPhone/iPod Touch chargers that only charge when the display is off. You can be using it for music with the display asleep and still charge.
I haven't heard of the brands/models of car chargers that give full 10W charging for use with an iPad (search the forums or elsewhere for specs, I guess). With the battery life it has, I haven't felt the need to ensure portable on-the-go charging when driving. I guess for a long road trip it would be very useful.