The iPhone 6's biggest drawback is its battery. It isn't a deal-breaker—I was able to make it through the day on a single charge. But when I stress-tested the handsets by cranking up screen brightness to 100% and streaming video, the iPhone 6 battery died just before my iPhone 5S's did.
The iPhone 6 Plus, with its larger battery, lasted about 15% longer than its sibling. But Samsung's Galaxy S5 blew away the competition, lasting 40% to 50% longer than the iPhone 6. (Plus, you can also replace the Galaxy battery any time. Apple's are sealed inside.)
In tests with all the phones' brightness set to automatic, the new iPhones were more conservative in their power usage and managed to match or beat their predecessor and arch rival.