I recently upgraded from a Radeon 3870 to the 4850. I uninstalled the original drivers and installed the new ones perfectly. It showed no signs of failing.
Once I started to play games (such as CoD:MW2 or Mass Effect 2) it would freeze after about 10 to 15 minutes. The screen would be frozen at whatever happened right when it crashed, and sound would then cut out after about 10 seconds, forcing me to restart.