Sharp as a doughnut and other similes

A few more:

Sharp as a bee sting
Sharp as the eyes on a hawk

Dull as a cafeteria meatball

Smooth as a ninja and twice as cool
Smooth as a smoothie

Normally used to describe someone's personality, but could apply to razors also: Dull as ditchwater.

my favorite has always been sharp as a brick...probably because both my brothers are brick masons, and I have done some brickwork myself.

Another that I think I heard from Foghorn Leghorn - "I say, that boy's as sharp as a sack of wet leather".

Or something I used to tell by brother in law "you could $^&* up a bowling ball in a sandbox with a rubber mallet!"