The MakerBot Stepstruder MK6+ is just what you'd think -- all the direct-driven, controllable, torquey goodness of the MK6, with an added plus!
That plus is a new, quicker-heating aluminum heater core, powered by a custom-made cartridge heater. In addition to faster heating performance, the MK6 Plus is easier to put together and has fewer points of potential failure.

For full instructions, see: 3mm assembly: http://wiki.makerbot.com/stepstruder-mk6-plus-assembly-3mm or 1.75mm assembly: http://wiki.makerbot.com/stepstruder-mk6-plus-assembly-1-75mm