Same here. I actually thought I had damaged the backcover clips when disassembling to retrieve a wayward nanosim to microsim adapter. Didn't realize it was the bulging battery until the random reboots finally forced me to buy a battery on the aftermarket.
The one downside I've noticed is that even with my "OEM" replacement battery (who really knows with these things), charging a lowish battery via a high amperage USB still takes significantly (multiples) longer than my iPhone 5 or 6.
The one downside I've noticed is that even with my "OEM" replacement battery (who really knows with these things), charging a lowish battery via a high amperage USB still takes significantly (multiples) longer than my iPhone 5 or 6.