I'll be running megasquirtII soon. I haven't started my engine yet, but I have built the Megasquirt and runs on the simulator board. I managed to solder it together and upload the firmware and base maps with little drama, but I wouldn't recommend that challenge unless you have a nice solder station and experience. The board is fairly densely populated.
What I like most about the system is it's not at all "black box", and there are additional outputs that are easily configurable. I'll use one of the extras to override my fan speed controller if temps get too hot.
I'm keeping the mech advance and distributor with a MDS 6A box to give a base curve and multiple sparks, but will also let Megasquirt "adjust" the timing for the vac advance and fine tuning. I will also have a breakout box so if the MDS box fails, with the flip of a switch, I can be back on the road with Megasquirt's coil driver.
I highly recommend
DIYAutoTune.com Megasquirt Kits / Assembled Engine Management Systems, Wideband o2 Sensor Systems and tuning products as a supplier.
But take this all with a grain of salt...it hasn't run yet.
I'll be watching to see how it comes out.