Generally speaking, there's two repairs on phones that people try themselves: replacing a cracked screen or a bum battery. On the M9, neither of those are really possible without a healthy dose of bravery, some know-how, and a lot of practice at Operation.
As iFixit found out, opening up the case isn't too much of a problem: just a pair of Torx screws hiding under the panel. But the motherboard sits on top of the battery, and is glued in place, making a changeover really freaking difficult. Even worse, the screen assembly sits below everything, so changing a cracked screen means disassembling the entire phone.
The crew over at iFixit also noted that their handset came fresh out of the box with a dead pixel and a scratch. That's not great news for HTC, which has always touted the One series as being the high-end, premium Android choice. [iFixit]