The guys over at Bit Tech have just reviewed the 5870 Voltage Tweak..
http://www.bit-tech.net/hardware/graphics/2009/10/20/asus-ati-radeon-hd-5870-volt-tweak-review/1
http://www.bit-tech.net/hardware/graphics/2009/10/20/asus-ati-radeon-hd-5870-volt-tweak-review/1