Love this movie. First saw it on late-night TV as a teenager. Found a copy on eBay a few years ago. Has a nice Hammery feel to it.
A couple of things stand out for me with this movie, most notably that our gentleman scientist, much like the Legion of Doom, is only allowed to use a plan once. The electric chair worked perfectly. It was controllable, easy to stop, and wouldn't necessarily kill quickly. It was a complete success. So, what does he do? Build a trick guillotine. When that fails catastrophically, does he return to what worked? No, he builds a gas chamber.
Of course, the gas was part of Giles' plan, but nobody seems to consider that they are jumping through a lot of execution devices.
Was that, perhaps, the movie's gimmick? You get to see people hanged, electrocuted, beheaded and gassed, not to mention blown up and smooshed between two colliding cars.
Did find it interesting that although Cunningham is immortal, he still ages. Assuming he recovers from the car accident, maybe he'll turn into the Face of Boe in a few hundred thousand years. That is, if he doesn't get his head chopped off by Clancy Brown in 1986. Actually, that would seem like a logical part of the process.