after 20+ hours of printing the core is fine i think this is a rather robust design
the major con it that it takes 50 to 10 min to get up to temp depending on conditions in the room
the picture is my core after about 20 hours of printing there is a 608zz bearing for comparison
The original made in this instructable lasted about 50 hours of printing. a new version made with 4 ohms of nichrome wire it heats up the MK 6+ heater core in under 2 minutes and has been printing since September 2011with no maintenance. i have printed off 4 full sets of prusa mendel parts and lots of miscellaneous printing. which totals more than 100 hours.