is the slightly better camera worth 300-400$ to you? if it is, buy it. I am 100% sure most of us would not.
Might want to spend some time over at the OnePlus forums to see the other side of the coin which is OnePlus support as well as software. OnePlus has basically left the One, Two and X phones as after thoughts. The One I can understand as it is the result of their pissing match with CM.
The Two and the X are not even a year old and the Two was originally marketed as their "2016 Flagship Killer". The X was really just a proof of concept to see if they could sell a subpar phone that was physically attractive.
The Three is a great phone with some software issues here and there but overall a great value. Just be aware that if you buy from OnePlus don't put much stock in support for the hardware or software in the future.
If you are comfortable rooting your phone and flashing custom ROM's then it should not be much of an issue. For the normal every day user, I would tell them to buy something else.
You can pick up a lightly used Nexus 6P on Swappa for under $350 most days and the price should drop some more as the HTC Nexus drumbeat gets louder closer to release.