I have two separate APU systems, and both have serious instability issues when C6 is enabled.
ECS A75F-M2 FM1 AMD A75 (Hudson D3)
2x2Gb Kingston HyperX 1600 RAM
Antec EarthWatts 500W PS
Windows 7 Pro x64
ASUS F2A85-M/CSM (Hudson D4)
2x4Gb GSkill Sniper 1866 RAM
SeaSonic SS-400FL2 400W PS
Win7 Home Premium x64
Both configurations have BIOS optimized defaults and no overclocking or any other such fiddling. I've updated both BIOS and AMD drivers when updates are available and have made no difference. If/when either system reboots or hard freezes, nothing is written to Event Viewer.
System 1 used to be my primary HTPC and would hard freeze in WMC. I had assumed these lockups were the result of a cheap motherboard and purchased System2. Re-purposing this system recently found it will not work at all with C6 enabled. I have attempted to install both Win7 and Win8, and both will throw the system into a reboot loop once Windows loads. It has been very stable with C6 disabled.
System 2 is an HTPC and hard freezes in WMC with C6 enabled. I have never experienced a hard freeze outside of WMC, but then again, its whole purpose for existence is recording live TV. There is no set amount of time before a hard freeze occurs; sometimes it happens immediately, other times WMC can run for hours.