well, I had a flex for 6 months,(actually 3 of them), and I did like the phone, even if the graphics(720P) were just so so; my last one had bad ghosting, but that went away after the 4.4 update.
I dumped the flex a few weeks ago for an M8, and I have to say I like this phone ALOT more; smoother, much better screen and sound, and tons more development, if that's what you are into. Also, endless accessories for the M8, and hardly anything for the Flex..
The flex battery is the king of all batteries, no doubt, but the M8 easily lasts me all day, so that's all that I really care about.
I thought I would have a problem with a smaller screen, but, in the summer, when shorts are worn a lot, its easier to keep my pants up with a M8 in my pocket than it was with the Flex
I am waiting to see what the Note 4 brings to the party, and also, to see if HTC comes out with a M8 MAXX; THAT would be my phone of choice if that happens, but who knows what the future holds.