I just used my ATT upgrade on an LG G4 last week. I would have loved to stick with WP, but the microphone on my 1520 hasn't worked in months — can't call anyone, can't use Cortana's voice feature. The G4 hardware is really, really nice, but Android is not.
I've read all the reviews here on /r/windowsphone where people say they switched back to Android and found it to be buggy/slow/poor, and I always dismissed those reviews as raving rants from fanbois. After all, I had a Nexus 4 before my 1520, and I loved it. I thought that Android was the bee's knees at the time, and nearly returned my 1520.
With this G4, though, all I notice are bugs and pitfalls. The day I got it, it literally installed app updates for 4+ hours; no idea why it didn't just download the updated apps in the first place. On top of that, I have to restart the phone every time it tries to update a Google app (which is all the time); if I don't restart the phone, it will just get stuck trying to update that single app and won't install or update anything else.
One thing that I was really looking forward to was changing the default SMS app. Android has a ton of really nice messaging apps, but so far I can't get any of them to work properly either. Notifications sometimes won't open, group texts never open, I can't send any MMS message, etc.
It's nice, I guess, that there are far more apps in the Google Play store, but WP actually has all of the apps I want to use except for the oft-reviled Snapchat. Material design is okay, personally it's a bit too colorful and fisher price for me — but the real problem is all of the apps that don't use material design; they stick out like a sore thumb. Not to mention every app has an endless hamburger menu and the 3-dot menu.
The worst part, what I really can't get over, is the lack of live tiles. Compared to the iOS and Android "grid of icons", live tiles are just superior in every way. I'm honestly surprised that neither Google nor Apple have copied Microsoft here.
As it stands, I'll probably be returning the G4 and just dealing with my broken 1520 until the next WP flagship is out. It's a shame, because the G4 itself is really pretty. I just wish it ran Windows.