I had the M7 for 6 months, the G2 for another 6 months, and now have had the M8 for just over a month. I was sad when I switched from Sprint to T-Mobile and had to give up my M7, but I did that in January, before the M8 was released, so the G2 was the next best bet for a phone. In my mind, I was really ready to go back to the M8 for reasons that aren't based on internals or on paper benchmarks, I went for purely cosmetic and usability.
The things I liked about the G2:
-Phone felt good in the hand - good size, a bit wider than M7/8
-The buttons on the back, were really nice once you got used to them
-Knock on feature works differently and better imo than on the M8
-I liked the setup of the stock keyboard (even though the word prediction was horrid)
-Battery life w/ battery saver was great. Compared to the M8 it depends on your OCD level. The % level on the G2 could stay at 100 with light use for hours, where the M8 drops pretty quickly. But overall in my normal work day, by the time I unplugged at 7am, left around 4pm I was between 70-80%, so the OCD need to be at 100% as long as possible people will like the G2 for that reason.
-It wasnt a Samsung. Granted it looked and ran similar, I loathed my Galaxy II and will never buy a Samsung Phone again.
Why I didnt like the G2
-From the moment I put my M7 down and picked up the G2, it felt thin, and cheap. I like a thin phone as much as the next person but it was so light, it felt like I could snap it in half.
-Sluggish UI
-Laughable speakers (coming from the M7)
Again the G2 is a perfectly good phone, but I went for the M8 because it felt better, more like a premium phone, even though I keep it under a screen protector and case all the time. I really missed Boomsound ... those speakers are just fantastic. The sense UI is also fantastic, feels very clean and quick. The size of the M8 is a bit interesting, as it is a tall phone. I didnt notice that much of a difference besides it felt a bit thinner, but it's still usable. Even though there are internal reasons for it, the Black bar at the bottom of the screen is not great, i would rather have had capacative keys there rather than nothing. Because of the metal it is a slippery phone, so a nice clear case does the job to make it more manageable. I am indifferent with the camera side of the phones. I really was not impressed at the 13MP camera on the G2, it was slow to take pictures and they werent special. The M8 takes decent enough photos, granted I dont use my phone as an everyday camera, minus sending pictures to friends or uploading on ebay.
Overall I was super stoked to get an HTC device back in my hands last month, I was excited to get rid of the G2 even though it was a perfectly good phone, it was just not my cup of tea coming from the M7. Yes the M8 has it's flaws but I have faith in HTC and I love their phone. Hopefully this rambling helps!