Macrumors reports the following –
A number of iPhone 6 and 6 Plus owners have been reporting an issue with the device that causes it to crash and remain stuck in a boot loop following a restart. The issue appears to primarily affect 128 GB models, particularly those with larger app libraries, and some suggest it may be a hardware problem associated with the increased storage available on that device.
The only recourse for owners experiencing this boot loop is to bring their phone back to Apple and request a replacement. Some in the industry point out that considering that technical defects mainly occur in the 128GB version of the iPhone 6 Plus, there might be a problem in the controller IC of triple-level cell (TLC) NAND flash.
As numerous reports of low read performance of the Samsung SSD 840 and 840 EVO using TLC NAND flash have surfaced on the Internet, a problem in the controller IC is considered to be the more likely cause of the defects.
The report says Apple could be facing a “huge potential recall” to address the issue, but the claim is purely speculative given that the prevalence of the problem has not been established and an exact cause remains undetermined.
All I can say that I have not experienced anything like this and have a daily backup of my phone. So should a problem arise, the beauty of an Apple product is a quick 10 min visit to the Apple Genius and then a full restore via iTunes to bring back your phone as previous.