I'm currently a Galaxy S4 user running a GPE rom. I've been fairly happy with everything about the phone EXCEPT the build quality. I'd been on the fence about what to replace it with (going on 1.5 years old), and the OnePlus One has been high on my list. My main concerns had been whether 5.5" was just too big. A friend of mine just got a LG G3 and, after playing with it (same width/thickness, just a bit shorter than the OnePlus One) I'm confident it won't be TOO big. I'll likely miss the hardware home button the most, but the double-tap wake will probably replace my main need for a hardware home button (waking while docked on my desk). I'll also likely miss the AMOLED screen as I use DynamicNotifications as a lock screen, which works better on AMOLED phones.
I'll probably be ready to pull the trigger right around when pre-orders open up (October), but if I get an invite before then I may be able to jump on it.