Artist here. I've tried both the Xacto and KUM sharpeners mentioned in this thread. Both are now long, long in the trash after dulling and becoming useless. What you want is a Mobius+Ruppert sharpener. They're machined from solid brass, their weight feels amazing in your hand, they're a beautiful tool, AND, most importantly, you can buy replacement blades. This was the #1 selling point for me. They're about $10 and will ACTUALLY, truly, sincerely last a life time. You can't break 'em. Hell, you could smash a windshield with one if you threw it hard enough.

I adore mine. It lives in a pretty cut crystal candy bowl in a nest of colored pencil shavings.