To begin, I wouldn't consider myself to be a hardcore phone junkie, but I like to upgrade every year or so, and my wife has always enjoyed my hand-me-downs. I don't root/flash ROMs, but that's primarily due to a fear of bricking my device, but I have considered doing so, particularly with my last phone (Note 2) primarily because of the absolutely RIDICULOUS amount of software bloat. I like Samsung, and the Note 2 was great, but holy cow....so much bloat!
Regarding the G Flex (on Sprint), I would like to try and dispel this recurring, rampant notion that the 720p screen is substandard. I just don't get it. I have shown my device to all of my friends and they are surprised when I tell them the screen is "only" 720p. The colors are vivid and the display is sharp. Video looks better on this device than on any other device I've used, which the curve definitely influences. Everyone who has watched video on the phone agrees...the curve, while subtle, definitely improves the viewing experience.
Speed is excellent. No slowdown or stuttering yet, but there is a weird effect when web-browsing sometimes...for a split second new pages will be very blurry (text in particular). Odd, and the only strange behavior I've noted thus far.
Keyboard is great. I used my Note 2 for over a year and CONSTANTLY had typos...even when I was trying to be careful. Capture is much better with G Flex and I need to correct far fewer mistakes. Back buttons are easy to use and are perfectly positioned, although are unwieldy to reach when I have the phone on a charging dock. Not a big deal by any stretch.
I absolutely LOVE the multitasking features. Clicking a link in an email will launch split screen, showing your email in the top half, and the other content (eg web browser) in the lower half. Why all devices don't automatically have this feature I'll never know.
Antenna strength seems comparable to my Note 2, or perhaps very slightly worse. Call quality is very good, earpiece volume is fine, and the speakerphone is sufficient for me.
Also, I have never had as many comments about a smartphone before as I have had with this. Multiple times in less than 2 weeks folks have said..."OMG...is your phone CURVED?" or something to that effect. It is, without question, a looker in the appearance department. The phone is dead sexy to look at...Samsung needs to take notes from LG's designs lately.
In all, I've been TRYING to find reasons to shoot down this device, but all in all, it is a great upgrade from my Note 2, although the lack of SD card capability was a drag. I have a couple more days to decide for sure, but thus far the only compelling reason I have to return it would be that LG devices don't hold their value like Apple/Samsung. I tend not to sell my devices though (my wife gets 'em when I upgrade), so thats not really an issue for me either.
Great stuff so far. And sorry for the long post..just trying to lend feedback for folks sitting on the fence.