That's a bit of a tangled one. Though no one has done experiments for awhile (at least not legally), there are a few camps on the topic just by reasoning through the problem. The current camps are:
1) Oh yeah, dude's dead.
2) It's possible, however unlikely, that the blade of the guillotine is so fast that some individuals do not pass out do to shock, so theoretically they'd be conscious until their brain runs out of oxygen in 6-13 seconds.