Devices with more capacity are available. I've got one that does 2A from one socket and 1A fro the other. Both will charge my phone.
But I have a problem with the stability of the voltage. Just charging the phone is great, but if I plug the 3.5" jack into the radio to play music from the phone at the same time as I'm charging, electrical noise from the car's electrical system gets through to the phone and renders any quiet audio un-listenable.
I'm just wondering whether I should fork out for a branded adapter, although the one I'm using was not a pound shop special. Anybody any idea whether Belkin et. al. actually make their adapters using better components, or whether they just slap their name on the same old tat and charge a higher price