Got my Chinese phone. The screen is very nice and it's responsive, but that's to expected I suppose. A lot of the kinks in the S4 are mysteriously no longer present, things like the battery getting hot with minimal use, random reception dropouts, less than 1 day on a new battery etc. Came with CM11 not ColorOS so I didn't have to flash it, and it let me quickly get rid of the Chinese bloatware.
It does seem a bit heavy. It's still lighter than most phones (interestingly the iPhone 6+ weighs 10 grams more) with a 5.5inch screen, so I'm probably just not used to using such a large phone. No expandable memory but I don't need it with 64Gb. It doesn't look like OnePlus make separate batteries for it, so I'm a bit concerned about getting long mileage from a Chinese battery. It's fairly easy to get the back off of the phone, so with some luck they'll sell replacement batteries eventually. Also has some Chinese scribbles on the back, but who cares?
Besides that it's amazing value for money. If you're interested and can get an invite for the International (US) version then you should definitely grab one, since they're even cheaper than mine.