His ability is the "Robo-Spin", which allows him to gain a bit more height at the start of a jump or make the descent slower. His ability also allows him to bypass fans easily as it sends him higher than any other character. He is basically the PC version of the Pink Knight.
Josef also tends to stick to walls. Repeated wall jumps allow Josef to maneuver in hard to reach spots or to skip certain parts of a level.
In a poll on the Super Meat Boy forums, Josef was voted as the worst character in either version of the game. Out of 125 votes, Josef got 29 votes (23.2%), nearly double of the character in second place (Headcrab got 15 votes).