I am in the EXACT SAME BOAT you are, OP. Die hard Android fan and definite flashaholic - custom ROMS are nice but not if you can't use all features of the phone.
Switched to the 6+ and am loving it more and more every day. No regrets!
I second this as well. I was an Apple guy up until the 4s. I wanted more from a phone and android was delivering what I wanted, bigger screens, removable batteries and expanded storage. I dabbled here and there with the ip5 and iP5s but nothing more than a few days at a time.
I've got just about every flagship android phone from every vendor. (Yes I know, that's incredibly costly but hey, that's what i Like). I've noticed over the past few years Samsung has grown stale with the S4, Note 3 and S5. Meaning the touchwiz interface really kills the experience for me. I was always downloading third party launchers etc.
LG has come a long way and my work phone is actually the LG G2 and wouldn't trade it for anything. battery life is outstanding and has a decent camera. I also have the LG G3 and while I'm mostly positive about the phone I do have some issues with it mainly the 2k screen. Not worth the battery drain honestly. 1080 screens are more than fine.
I have the HTC One M8 and it's a well designed phone except for a couple of flaws that bugged me. First, the length of the device. The bezels on the phone really detract front he beauty of the phone. It's just as actually taller than my LG G3 and it's screen size is half and inch smaller. There is definitely room for improvement. The second issue i have is with the 4 Ultra Pixel camera. The camera is capable but really has issues with bright objects and off color reproduction. Not to mention HTC still is using plastic camera lens covers that scratch with ease. I'm on my second M8 because the first one, the camera lens cover cover scratched even while being in a case. it looked like sandpaper hit it.
If you do a google search, there are a ton of people that had the same issue. IMO, if you're going to be build a quality device, then you don't skimp out. Software wise, hardware wise and build quality wise..
That's why I ended up with the iPhone 6+. It got almost everything I wanted, right. The only thing I don't like is the fact that it's bigger than my galaxy note 3. Outside of that, Apple nailed it. That's why i'm back on the Apple mothership.
Of course....i still have plenty of Android devices to play with when I get the itch. This will be the first time that I pass up getting a major release phone when the Note 4 comes out. I will be waiting to see what Google Drops with the Nexus 6 though.