had a chance today to actually check out the m8, first impressions....simply gorgeous!. the m8 was displayed right next to the sg s5. simply no comparison imo looks wise and build quality. but first and foremost i wanted to try out the camera, all i can say that when compared with the nexus 5 which i played around earlier at tmo and the new sg s5 is that for the most part it was decent at best if and only for social media.but once you start zooming things in and cropping thats when you start to see the difference in detail, that being said i have concluded that it is NOT a dealbreaker when compared to the other exceptional atributes the m8 has to offer and i also think the camera doesnt deserve the slamming its got. 
In my limited time with all 3 phones comparing cameras
 
M8- crazy fast shutter (important for catching the moment) 
         good low light capture ( but loss in detail )
         shooting in sunlight sky on most shots look overexposed 
         HDR+ mode nothing special
         no ois, but software image stabilization is decent
         great camera UI
GS S5-fast shutter 
             megapixels do count especially when cropping and blowing things up a bit
             good camera UI ( but a bit too busy)  
Nexus 5- slow in HDR+ 
                 always good to have OIS
                acceptable cropping and enlarging to a certain extent
                 vanilla ui
For me it comes down the to N5 or M8...I will be buying off contract so price comes to the equation, but i gotta say despite the camera being the only thing i cant aplaud, the M8 is truly something to behold.