I'd put it about at 'Happy Tree Friends' level of amusing.
In other words not. I'm still baffled how people laugh at that...I can play a video game with violent graphics but make it all cartoons and I feel so dirty...it's like something sacred has been violated...too close to watching my beloved childhood Saturday morning cartoons in an R-rated context...just so wrong.
It's actually the epitome of a joke.
A joke is two stories. The first story (instance, situation) gets you to make an assumption as to what will happen. The second story totally flips that on you and thus makes it incongruous. All humor requires incongruity.
The anti-joke is the opposite but can still be funny. The prime example is "Why did the chicken cross the road?"
"To get to the other side."
This joke requires that the person knows it's a joke and for him to make an assumption, and as such when hit with reality it causes the incongruity.
However, all humor is at someone or something's expense and is subjective. So not everyone finds it funny.
And as E.B. White said, "Dissecting a joke is like dissecting a frog: Not many people like to do it, and both are dead at the end of it."