I'm actually having trouble getting my 64GB production unit to boot into TWRP or have it flash and stick. fastboot says everything is fine, then rebooting into recovery produces the stock simple recovery.
fastboot boot twrprecovery.img ends up hanging on the fastboot screen, basically nothing happens, but fastboot does stop responding to commands.
I'm sort of at a loss - I've done this to many Nexus devices before, and the idea is the same. My bootloader is unlocked, that went without any issues.
Booting a debug boot.img from
https://cyngn.com/products/oneplusone via fastboot just ends up shutting down and showing the battery charging icon.
Volume Up + Power while booting ends up with a black screen - in fact, the only way I can get into recovery is from a booted OS.
Edit: After many tries, trying different cables and unchecking the Settings -> Development -> update recovery with system updates, the TWRP flash succeeded, and the subsequent SuperSU sideload went just fine. Now I'm rooted.
Another tip regarding using Power+VolDn or Power+VolUp - things happen so slowly that if you keep holding Power, it'll actually trip a reboot before it has a chance to load. Just hold the Power button for a few seconds when booting, then let go and keep the VolUp or VolDn pressed.