I've had it once
I was in a rush trying to get my phone to work (bad SIM from AT&T, it seems. The guy, when I finally got served after an hour, asked me what ccid or whatever I had, so I had to find it by scrolling rapidly, four or five times, slightly diagonally, and it froze, solid. Couldn't get it out of the crashed state -- I suspected it was a software crash. The physical button got me out of the System Preferences and I relaunched.
Degree of annoyance? Never had it since. AT&Ts crummy SIM card and their lack of an LTE tower in my part of the city are way more annoying. I still haven't experienced any LTE speeds. I love the phone, in my area Apple Maps is pretty cool, when I go to an unfamiliar place it hasn't lost me yet. Oh, and I can use FaceTime over cellular, but only if I leave Unlimited. I very seldom go over my time, and I got my first warning of slowdown last month, though never a slowdown. What they want is to ream you on "over the top" fees. They'll give you FaceTime, but you'll pay for it all.