Just purchased an M8 through my carrier 10 days ago or so, coming from a Nexus 4.
1st unit I got ; the first thing that came to mind when opening the phone ; "Why the heck would they choose a yellow background for the splash screen / HTC logo background? Oh well, carry on." ... Only to realize afterwards that everything was yellow. And dark. And impossible to read outdoors with max brightness on. And the volume rocker was rattling, as described previously in thread. Serial # of unit was SH45NWM*****. Returned the unit asap. I wish I took a picture of a white test screen on a Nexus 4, Xperia Z3 and my HTC One M8 next to one another ; it was ridiculous to see my 2 years old, 300$ Nexus 4 outperform the M8's screen in terms of white balance and brightness honnestly. Xperia Z3 was a bit on the blue side but SOOOO much brighter and crisp as well.
2nd unit I got ; screen was a BIT better than the first one, but then again not really but I figured I'd live with it. as this is probably what the screen looks like and I could live with the 2nd one, it was a tadbit brighter but still definitely yellow. Same thing for the loose volume rocker ; figured that with a case on it wouldn't matter. And then after just over 1 day of use, started a Youtube video at 90% volume or so and top speaker "popped" (for lack of a better word). Constant rattling after that when volume is 75% or higher. Just returned 2nd unit today. Didn't think about taking a snapshot of serial # but it was another SH45*******.
Side note ; I also recently ordered a Nexus 9 (HTC built of course) on day one of its release, only to return it after 2 days of use. The build quality was absolutely ridiculous (I was one of the lucky ones who got creaky back, very noticeable light bleed and subpar battery life.)
I'm giving the M8 a 3rd (and last) chance as I personally think this is the best looking and sounding phone on the market right now, if it doesn't work out I'm turning to Sony or Motorola and will never give HTCs another look. I've never returned any laptop, tablet, smartphone, or such for build quality issues and for HTC, it's been 3 in 3.