The only drawback to the guillotine is that the head apparently can still remain aware for another 5-7 seconds after decapitation, which can't be fun.
Still, if you're executing somebody justly, you're not trying to make it a fun experience. Perhaps as with little pain as possible, but there's nothing fun about being executed even if it's entirely painless.
And it should be fun? ;)
I guess 5-7 seconds can be an eternity (so to speak) for the head, but do we know if there is pain and do we really care?
Not trying to be augmentative, just a personal thought.
To me this would be the most humane way to execute someone … in most cases, the amount of pain that the condemned suffered would be far less then the pain and fear that they caused to the victim (and maybe of shorter duration).