I just assembled the MK3 alu heatbed. It seams to work good.
I placed a relay on the power line to the headbed instead of power it directly from the RAMPs. I read that this bed sometimes can take a little bit to much current from the RAMPs. I placed a 40 A 12V realy to a car.
The NTC that was coming with the heatbed was a 100k SMD that you mount directly on the edge of the heatbed. No NTC in the middle hole. It´s working good. The only sad thing is that when i build the printer from the beginning the mounting holes in the y carriage was drilled as M4, The headbeds are only M3 so I have to use a nut instead if the original threaded holes.