When the battery drained, it refused to boot past the "cyanogen" boot screen. Twice. A common problem that required a factory reset.
Would say it's charged to 100%, until the moment the USB was removed and it instantly went to 4%. Every time, no matter whether it had been drained, or charged when turned on/off. Again, factory reset fixed it.
Wont vibrate more than once with the OnePlus official flip cover when calls are incoming on silent mode.
WhatsApp vibrations cant be turned off since CM12 upgrade.
Doesn't do 4G in Europe. There's probably a carrier whom it does work with, but from my last bit of research there isn't in the UK. Perhaps Three though.
I've used CM happily on other handsets, and encountered far fewer and far less frustrating. However if you're marketing a phone specifically to be used with CM, I'd expect a lot more QA to be put into it. If you're happy with a "hackable" phone and you're willing to put up with missing calls, disappearing notifications, private lock screen notifications showing message contents still, then it's not a bad way to waste £300, but I want a reliable phone I dont have to fight with, so I'm sacking it off very soon.