A recent blog post by software engineer Ken Shirriff tested a dozen USB chargers sold worldwide: both the versions that come in the shrinkwrapped box from the manufacturer, as well as third-party products. It also looked at counterfeits. The article asked the question whether "a charger provides silky-smooth power or if it is a dangerous charger that emits noisy power that cause touchscreen malfunction and could self-destruct."
The article offers a wealth of detailed information on chargers, including tear-downs, photos, voltage quality tests and frequency plots. It also looked at whether the charger delivers the advertised power. Some hit the mark, while others were way off.
The counterfeit chargers perform extremely poorly, putting out a fraction of the expected power. Charging your device with one of these chargers will be a slow, frustrating experience. In particular, the counterfeit UK charger only produces a third of the expected power. Although the label claims the charger works on 100-240 volts, it's clearly not designed to work on US power.
Shirriff noticed a voltage sag issue with the iPad charger. He found it puzzling.
Similar to the iPhone charger, the iPad charger shows a lot of voltage sag. The voltage is about 5.1 V unloaded, dropping to 4.4 volts and 2.3 A (10.1 W) at the corner. Unlike the iPhone charger, the iPad charger has pretty good current stability. The regulation is solid, as shown by the narrowness of the yellow trace. Note the scale change due to the high current output.
I'm puzzled by the steep voltage sag on both the iPhone and iPad charger. Since the designers of the Apple charger went to a great deal of effort to build a high quality charger, I conclude they must not consider voltage sag worth worrying about. Or, more interestingly, maybe they built this sag as a feature for some reason. In any case, the chargers lose points on this.
He also suggests that users should make sure to use the correct charger for their phone model; for example newer iPhones.
Perhaps consider CableKeeps by Nice By Design; I use them with my own power adapters. These covers slip over the adapter and look like little blue, green and orange fish. They let users wrap-up the cable for storage, identify the adapter and more-easily pull the plug out of the power socket.