Well I made the transition recently. The M8 is really a beautiful and largely amazing phone.
Two major issues made me leave it behind:
1. For me, the screen was not large enough. Even though it is 5", it's a narrow proportion screen, like the iphone 5 (only overall larger obviously). Obviously for someone used to an iphone 4, it's a big phone. All personal preference.
2. The camera was inferior, to me anyway. I found it was taking lousy pictures that were very unprofessional for work use and disappointing for personal use. No way around that. It's HTC's stupid Ultrapixel approach to the sensor. I hope they change this completely for the next iteration.
Other than that it's great. If it had a better camera and a 5.5" screen, I wouldn't consider the G3 for an instant. The 1080 screen is beautiful and perfect in every way- to me. Very smooth ui and operation. Very nice phone, no way around it. Speakers are great. Also, as I'm finding out, it's nice to get this direct from HTC as unlocked (not tied to a carrier). I got the Developer Edition which as well as being SIM unlocked also has an unlocked bootloader. It's very nice not to have all the bloatware, clutter and crippling that AT&T provides on their phones. Unfortunately the G3 is not currently available as a non-carrier-branded phone.