Ok, next stupid question... What is a kernel compared to a ROM? So the big advantage of the Franco kernel is to help with battery life?
If your ROM is the car you are driving, the kernel is the engine. The android kernel is linux based, and it is the link between the firmware (ROM) and hardware (Phone parts: antennas, screen, sensors, etc). Any time any piece of the software wants to get information from or send it to a piece of hardware, that information has to pass through the kernel. Custom kernels let you over and underclock your CPU, alter the amount of voltage being used, set up power profiles, and most of the nice ones now let you enable hotplug (turns off 1 CPU core while the screen is off to further save battery juice), alter the color and gamma settings on your screen, and more.
A well made kernel can definitely increase your battery life, improve speed, make graphics and other operations smoother, etc etc. So to answer your question, Yes franco's are generally agreed to be really nice on battery, but that's selling them short.. I also find them to be very very smooth, stable, and there are a lot of options you get to toy with from within his app which is worth the investment.
Final note on kernels: every phone is different, not all silicone is created equally, and some people will find that some kernels agree more with their phone than others.. so you might not see the same exact results with each kernel that other people are claiming, and that's normal.