If you're looking to boost the charging speed of your new iPhone 6 or 6 Plus, the folks over at iLounge found that the new iPhone models can take a 2.1A charge -- like what you'll find in iPad chargers. According to some preliminary tests, iLounge found that both models can charge faster using a 2.1A supply, even though they ship with a 1A power supply.
Forum members over at MacRumors found similar results. Assuming the new iPhone models can actually take in the full 2.1A charge, this makes sense, since the added juice can actually be used to charge quicker, unlike on older iPhone models that will only take the 1A regardless of which adaptor you use. Either way, if you have an iPad charger sitting around, give it a try, there's no risk to damaging your iPhone.
iPhone 6, 6 Plus support 2.1A charging, ship with 1A adapters [iLounge] iPhone 6 [Macotakara]