The issue has been very significant on realms. I purchased a realm and the rendering has lacked large sections nearly everywhere. I have run into mountains or walls that weren't there until I hit them. Outside of that, I cannot see much distance as everything is broken in to random chunks on the horizon. It is discouraging. The problem doesn't seem to exist on single player.
ATI HD6800 also effected, using latest 14.4 drivers (about to test beta 14.7RC).