Picked mine up yesterday, had purchased the Icon, but when I went into the Verizon store, I found that I really liked the M8 better.
Surprisingly, the IPS display on the M8 is actually superior to the AMOLED on the Icon to my eyes. Considering I almost always prefer the color pop and super dark blacks of AMOLED, this IPS panel is amazing. Not quite as black, blacks, but the color reproduction is fantastic. The one thing I miss from the Icon is the Color profile settings. The M8 only has adjustments for Brightness, and as usual its Auto, Low, Med, and High, no sliders. Unlike the Icon, the Auto setting works really well, and never seems to go to dark.
As everyone else has said, the all metal chassis is fantastic looking, and feels about like a river rock or pebble in the hand. In fact it can be pretty slippery if you have wet or sweaty hands. It is also noticeably lighter than the Icon, same width, and about 1/4" longer due to the Boomsound speakers.
Ahhh, those speakers, man what a difference. This was one of the main deciding points for me on this phone. Unlike most folks around here, I really don't use the camera much, other than to snap a whiteboard here or there, so it made no difference to me. The sound however really matters, as I use speaker phone on and off all day, and listen to music the rest of the time. This thing sounds amazing. Deep, rich, full sound, and there is even enough separation between the speakers that you get a full stereo effect when in Landscape.
The power button on the top of the phone takes a little getting used to, but it works pretty well. The on screen navigation buttons are no big deal, and give you several options on how they operate. Slide up to show, slide up to hide, auto hide and show based on actions, and hit a small arrow to hide, and swipe up to show. Pretty easy to setup the way you like it.
Battery life is better than the Icon, I'm still testing, but so far this phone came off charger last night at 10 pm, its now 1 pm, 15 hours later, and I have 82% battery, even after installing a bunch of apps, listening to a lot of Pandora, taking calls etc.
The SDXC slot is a fantastic addition, I keep all my music there now. This phone is unlocked, and will work with LTE on Verizon, and T-mobile. AT&T however will only get 2g/3g, no LTE.
Another interesting thing I have noticed is that the M8 consistently has 1-2 bars better signal strength than the Icon here in Dallas. WiFi and Bluetooth too have much higher signal strength, than the Icon. Not sure if that is due to better hardware, better antenna, or the all metal chassis, but there it is.
I use the proshot app with the camera, and as far as functionality goes, it looks to be the same with the same options. Blinkfeed, is something that was sorely needed for Windows phone. Since flipboard has yet to bring an app to WP, blinkfeed does a really good job of aggregating my news, articles, social media in one easy to read place, Love it.
As for performance, it is noticeably quicker than the Icon, especially when restarting, opening large apps, etc.
Overall, this is a fantastic device, and really will come down to what features/functions are most important to you.
If your a camera geek with a need for a high end phone camera, the Icon is the way to go. If you prefer the design, signal strength improvements, and blinkfeed additions, then give the M8 a look, its one hell of a nice device.