Ken tested a dozen URB chargers, ranging from Apple's official USB charger, to brand name accessory maker Belkin, to tech bargain lover's go-to supplier Monoprice, and even a few counterfeit knockoffs. Each was rated by: efficiency, delivery of advertised power, power quality, and power curve (voltage sag, current sag, regulation); lots of figures and graphs if that's your thing.
But to cut to the chase, Ken's sample of chargers reflects the rational assumption sticking to original manufacturer accessories is the safest bet, while big band accessory makers offer fair compatible options, while eBay or other discount counterfeit knockoffs should be avoided at all costs for safety of your device.
Counterfeit chargers pose a safety hazard as well as a hazard to your phone. You can buy a charger that looks just like an Apple charger for about $2, but the charger is nothing like an Apple charger internally. The power is extremely bad quality (as I will show below). But more importantly, these chargers ignore safety standards. Since chargers have hundreds of volts internally, there's a big risk if a charger doesn't have proper insulation. You're putting your phone, and more importantly yourself, at risk if you use one of these chargers. I did a teardown of a counterfeit charger, which shows the differences in detail.
I've personally seen similar performance/quality issues with knockoff accessories for laptops and camera charging accessories. I'll never again cut corners when purchasing a laptop battery replacement; as the 50% off price seemed to come along with a 50% charge/performance drop off. Not to mention the usually shoddy manufacturing, something that could lead to a fire in certain circumstances.
(Images: Gregory Han; charts by Ken Shirriff)