Even though the Butterfly 2 trades in the aluminum chassis of the One M8 for a polycarbonate exterior, GSM Arena says it's still "well-designed" and Android Central calls it "one of the better-looking all-plastic smartphones out there." However, GSM Arena also feels it "lacks that special something" and a matte grip would have been more "sophisticated" with a better feel. The glossy plastic does have a distinct advantage, though, in that Android Central finds it "considerably easier to hold," which is a definite improvement over the M8.
The screen stays the same, with Android Authority calling the viewing experience "almost identical" to the HTC One though Android Central notes it has a lower limit for brightness, making it "too dark" for some indoor situations. They also say it "seemed to match" the battery life of the M8, offering "solid performance" though GSM Arena notes it doesn't improve on the Butterfly S.
Where the One M8 does represent a clear uptick in quality is the camera, offering a wider dynamic range, improved HDR capabilities and reduced noise. Android Central says it's "pretty good across the board" and phoneArena finds it "more formidable." But, even though the camera is better than the one on the M8, Android Central says it "continues to lag behind the latest" phones from rival manufacturers, and Android Authority says that "issues with image quality that have plagued previous HTC smartphones rear their ugly head here," including a lack of detail and color.
With all the incremental changes from the One M8 to the Butterfly 2, there isn't much to strongly recommend one over the other, making it largely a matter of personal preference. If you need your phone to look and feel premium, best to stick with the M8. Or, if you absolutely need a better camera and a plastic construction doesn't bother you, the Butterfly 2 is the HTC phone to pick up.