From its first use, there has been debate as to whether the guillotine always provided as swift a death as Dr Guillotin hoped. With previous methods of execution, there was little concern about the suffering inflicted. But where the guillotine was invented specifically to be “humane,” the issue was seriously considered.
Furthermore, there is the possibility that the very swiftness of the guillotine only prolonged the victim’s suffering. The blade cuts quickly enough so that there is relatively little impact on the brain case, and perhaps less likelihood of immediate unconsciousness than with a more violent decapitation, or long-drop hanging.

Audiences to guillotinings told numerous stories of blinking eyelids, moving eyes, movement of the mouth, even an expression of “unequivocal indignation” on the face of the decapitated Charlotte Corday when her cheek was slapped. Anatomists and other scientists in several countries have tried to perform more definitive experiments on severed human heads as recently as 1956. Inevitably the evidence is only anecdotal. What appears to be a head responding to the sound of its name, or to the pain of a pinprick, may be only random muscle twitching or automatic reflex action, with no awareness involved. At worst, it seems that the massive drop in cerebral blood pressure would cause a victim to lose consciousness in several seconds.[3]

The following report was written by a Dr. Beaurieux, who experimented with the head of a condemned prisoner by the name of Henri Languille, on June 28, 1905:

“ Here, then, is what I was able to note immediately after the decapitation: the eyelids and lips of the guillotined man worked in irregularly rhythmic contractions for about five or six seconds. This phenomenon has been remarked by all those finding themselves in the same conditions as myself for observing what happens after the severing of the neck…

“I waited for several seconds. The spasmodic movements ceased. […] It was then that I called in a strong, sharp voice: ‘Languille!’ I saw the eyelids slowly lift up, without any spasmodic contractions – I insist advisedly on this peculiarity – but with an even movement, quite distinct and normal, such as happens in everyday life, with people awakened or torn from their thoughts.

“Next Languille’s eyes very definitely fixed themselves on mine and the pupils focused themselves. I was not, then, dealing with the sort of vague dull look without any expression, that can be observed any day in dying people to whom one speaks: I was dealing with undeniably living eyes which were looking at me. After several seconds, the eyelids closed again[…].

“It was at that point that I called out again and, once more, without any spasm, slowly, the eyelids lifted and undeniably living eyes fixed themselves on mine with perhaps even more penetration than the first time. Then there was a further closing of the eyelids, but now less complete. I attempted the effect of a third call; there was no further movement – and the eyes took on the glazed look which they have in the dead.”

This interesting tidbit comes from Wikipedia, the free encyclopedia.