Just posted my review of this phone of engadget
I've owned a Nexus 5 for the past 5 months and recently sold it to upgrade to the GPE HTC one M8 and i couldn't be happier. This phone surpasses the Nexus 5 in nearly EVERY aspect (with the exception of size and weight) that to even entertain the notion that the Nexus 5 is somehow a "better" device would be an excercise in delusional frivolity.
Lets Start with Build quality. The HTC One actually feels like a solid, well constructed piece of kit. Furthermore it is an absolute delight to hold, and always feels cool and comfortable to hold. (Contrast this to the shoddy plastic casing of the Nexus 5, whose white variant picks up dirt better than an overcharged roomba) Also, the well constructed solid aluminum casing feels sublime, and the smooth edges really do improve comfort when spending a long day using your phone for completing the important tasks it was intended to do, like reaching level 200 in Candy Crush

. There really honestly isn't a need to sport a case due to its high build quality. Contrast this to the shoddy construction of the Nexus 5, with its light, almost ephemeral build quality. Honestly a strong wind would easily blow your nexus 5 out of your hands, resulting in undoubtedly cracked screens and hurt feelings.
Performance is about equal on both phones. Nexus 5 never lagged, but apps did crash here and there. I blame this on the shoddy OS that is Android and not the hardware.
Audio and Video performance is phenomenal on the HTC M8. And is literally the difference maker in what puts this phone in a class of its one. There is truly nothing better thanhaving true stereo sound on your phone. It makes playing games and watching movies a far more memorable experience. Paired with MOGA One Pro controller, this device literally becomes the ULTIMATE portable gaming solution as well. I currently have a plethora of PSX games, SNES games, Music, and Movies stored on my 64 gig micro SD memory card. So not only do i have an amazing array of entertainment options to choose from, none of them affect my precious onboard memory. This is something NOT achievable on the Nexus 5 due to its lack of expandable memory. (A huge misstep by Google in my opinion.)
Lastly, the special features of the phone provided by HTC also make this phone a unique treat. Not only do you get the benefits of a Google Play edition phone (ensuring timely software updates from Google), you also get access to all of the unique software features from HTC. So i not only get the vanilla Google experience the makes the Nexus line such a great group of phones, you get those awesome features that HTC has implemented in to their device like the swipe features, the duo camera, and the powerful photo editing soft ware
Now cons:
Yes the phone is bigger and heavy. But its weight issues are neglibile. Anyone currently with a lighter phone is generally obligated to sport a case negating any real weight advantages you previously enjoyed. While i still choose to sport a case on my HTC One M8 due to my obsessive desire of keeping my phones in mint condition, its sturdy construction and attractive form make this phone a far superior option than the "budget" phone du jour, the Nexus 5.