According to iFixit, it's basically a fool's mission if you want to open up your HTC One. iFixit says that it is nearly impossible to open the device without damaging the rear case, but the display can't be replaced without removing the rear case, making it nearly impossible to replace a cracked display by yourself. Additionally, a big problem seems to be copper shielding, which covers the motherboard, rear camera, and more.
iFixit says that copper shielding has benefits for dissipating heat and providing electrical grounding, but it "is a pain to deal with during reassembly—kind of like trying to straighten out aluminum foil once it's been crinkled."
So, if you're someone who likes to do your own repairs on a broken smartphone, the HTC One probably isn't the device for you. Of course, if you don't care about that, you may well find the HTC One to be quite a nice device, as we stated in our review.