Info Cameron gives is true, but, they are not as rare as they once use to be. Chrysler is reproducing them again, but, they may as well as be rare due to the price for a complete assembly.
Hemi wrote the book on making power, and due to its combustion chamber shape, makes it very efficient, all other things being equal.
It is the King!! Just listen to a Hemi and anything else at the starting line of your local drag strip, The Hemi's STILL pumping iron at the big end!!!!!