I upgraded from the M7 to the M8 and you can count me as disappointed. After two weeks, I broke my M8 (no fault of the phone) and went back the the Sprint store to order a new one, then have them re-activate my M7 while I wait for new M8 to come in. Using the M7 again was like going back to an old friend that actually works.
IMO, the M8 is not much if an upgrade at all and if you have the M7 and are happy with it, I do NOT recommend the M8. Here are my issues with it which I demonstrated at the Sprint store today.
Larger screen: ?? Don't be fooled. The effective useable length of the screen is the same size as the M7 although it appears to be about 1/4" longer. Why? Because unlike the M7 where the Back and Home buttons are on either side of the HTC logo at the bottom of the handset (and off the screen), on the M8, they are on the screen, along the bottom, using up exactly all that additional length at the bottom of the screen. The the useable part of the screen is a tiny bit wider, like an 1/8" but no longer and the pixel count is the same which means the overall resolution, or dots per inch, is less. But not so much that you really notice it. It just isn't an upgrade or any real improvement over the M7.
Those buttons: Again, where the Home and Back buttons are always available and lit up at the bottom of the M7, it requires a careful tap at the bottom of the screen to call them up on the M8, then an additional tap on the button. This is NOT an improvement, it's a downgrade in functionality that I find very annoying. Be careful when you are in camera mode and need the buttons. It's takes a very precise tap to call them up and not actually snap a photo instead if you have the camera set to snap on screen touch.
Screen brightness: First thing I noticed is in full sunlight, while driving, I could hardly see the screen when set to either automatic (which adjusts to full brightness in sunlight) or manually setting to highest brightness. I finally figured out that I had to turn off the battery saver to get that last level of brightness that makes it actually readable in the sun. On my M7, the battery saver is nearly always on and brightness set to automatic and it works flawlessly. It also dims the screen just the right amount at night where the M8 is too bright at night.
Stupid camera tricks: Again, don't be fooled. This is the same 4 MP (OK, "ultra" pixel) camera as on the M7 and overall, the shots look the same. What is different? They removed OIS (optical image stabilisation). They are saying it's really no difference since this fancy, two lens camera on the back is just so much better that you won't even miss it. BTW, it makes no sense to even compare those two things for they have nothing to do with each other. The two lens system does nothing to compensate for OIS and what it offers to hand held photography. They wanted to introduce this two lens camera so bad, which does not work with OIS, they said, who needs that silly old OIS anyway? OIS is a HUGE feature to take away, while most phones are just getting it, and a significant downgrade to the overall camera and video functions. We lose OIS for a stupid trick (that barely works most of the time) to either selectively focus a shot after it's taken, or blur the background like you used an SLR with a narrow DOF. Doesn't work that well and, even if it were flawless, I'd hardly ever use it. Most people will hardly ever use it but they traded OIS for this editing feature? Really dumb. BTW, the front facing 5 MP camera is really no better than the front facing camera on the M7 which is only used for selfies anyway. Again, no big upgrade.
More on the camera: Takes longer to focus and fire the flash in low light. Then the new dual flash is so bright that the pre-flash first stuns your victim, errr subject, making sure she is squinting in pain in the final photo capture. I've read where this camera is faster. Not in my experience.
Speakers: Yes, they do seem a bit louder than the M7. But do you seriously listen to music on cell phone speakers? Even the best cell phone speakers on the market can't rival a little $20 Bluetooth portable speaker. It's a cute trick but there will never be any bass coming out of speakers that tiny. OTOH, this is, hands down, the best sounding phone I've ever had for phone calls when held to my ear.
Other quirks which may be just my handset:
The notification sound for texts kept turning itself off while in my settings the slider bar was still set to loud. The only notification I got was to set it to also vibrate. The only fix was to re-boot the phone, then it would happen again shortly. The Sprint guy said they are seeing that with some phones.
How did I break it? I took it in because the microphone stopped working. A caller could not hear me at all. But I think I caused that by jamming my ear bud jack into the plug when there was some pocket fuzz in there. I could not get the earbuds to work and remembered pulling some lint out of the hole earlier. So I went at it with the surgical precision of the fancy plastic toothpick. I was able to remove more fuzz but I think some remained and the damage was done. Of course in normal use the earphones mute the speaker, allowing you to use the mic provided with the earbuds so I probably jammed the socket open (or shut?) by trying to force the jack into the phone.
Sprint insisted I take their $13 a month insurance when getting the M8. I could only cancel it myself, not refuse it in the first place. So I'm covered and will be getting a new phone in a few days. I'll probably ask them not to activate it and keep using the M7 until I break it, which I know I will. Then I have my M8 as insurance instead of paying $312 over 24 months to insure a phone that will barely be worth that much 12 months from now.
BTW, it's not lost on me that the M8 has a faster processor and a memory slot that will take a (very expensive) 128GB micro SD card. That is cool but I'm doing just fine with the 32GB of memory on the M7 and it's always been fast enough for me. I'm not a gamer or running any other CPU intensive applications.