It, like the other ignitions, increase the power of the spark. I have the Jacob's ICE Pack, and it claims to increase hp by increasing the power and frequency of the spark, meaning it sparks more, like 3 times, for every 1 time of the stock system. It also claims to burn away deposits and such and provide better fuel economy by completely burning the gas/air mixture each time.
They do help in the higher rev range where the stock system can't keep up. My car had alot of miles, and I had done several other mods, and it seemed like upgrading the ignition might help, or at least ensure my ignition was up to the task.
I paid close to $350 for my Jacob's with the plug wires. The install took me about 3 or 4 hours. Mounting the equipment is the hardest part...finding room and all.
MSD vs Jacob's...it's all preference. I like that the Jacob's comes as a kit, and when I looked at the MSD, I would have had to piece the kit together myself. And the ICE Pack is aimed at little import 4 bangers.