Long story made somewhat short, after my first experience
sending in my M8 for repair, the battery wasn't connected properly and the phone wouldn't charge. I once again sent my phone back, they repaired it by replacing the entire motherboard, and I received it nearly three weeks later. The battery still wasn't working.
I was leaving for Europe in a few weeks, but I sent my phone back a third time. HTC Support called me every day, sometimes several times, to alert me on my phone's status. I begged for them to make sure it was properly fixed. They assured me my phone would be thoroughly tested. They kept it an additional day after it was repaired to specifically test everything, especially the battery. I received it two days before I left for Europe. The battery was still disconnected. The phone would stay on while charging. I could use all USB functions. But the phone acted as if the battery didn't exist.
I pleaded with HTC and Sprint to help me out, to no avail. Sprint told me it was HTC's problem. HTC told me they were out of new M8s... I finally sent an email the President of HTC USA, Jason Mackenzie, explaining the issue and telling him that I was leaving the next day for a few weeks in Eastern Europe. He responded within minutes and overnighted me a brand-new M8 that arrived at 8am the next morning.
I was fed up with dealing with the daily calls and promises HTC made me over a several month period, but everything was made well in the end. HTC's customer support was top-notch, the repair process miserable.