I'm using a carrier-free 8x since April. After the GDR2 update (which supposedly causes huge problems and was pulled) there are no issues with the phone. It may occasionally lock up and reboot, but the frequency of this happening is about the same as for Android - about every 2-4 weeks.
Before GDR2 the phone would typically reboot every 1-2 weeks. But in any case, it doesn't lock up when not in use, the problem only appears under heavy usage so it's unlikely the phone will shut down silently in your pocket and prevent you from receiving calls.