MSD takes the stock module out of the system therefore eliminating the high RPM problems with HEI. You need good plug wires and a better coil to realize all the gains from an MSD box though. Wires are a must, coil is not 100% necessary but does help.
I ran MSD box, coil, wires on my truck with a carb. I DID see a fair gain from it. I would recommend it for a carb truck but not an FI truck.
The MSD box throws a hotter longer duration spark to help the motors combustion process happen more completely. More complete combustion will gain you MPG's and a seat of the pants HP.
In my experience the MSD box did everything it advertised. Easier starts, better MPG's, more felt power, etc. I was very pleased with it. I would run it again on a carb'ed truck. My roommate added it to his 66 Chevy with a stock coil and decent wires. He felt gains too. It starts right up and it pulls better too.
Harley