The knuckleball is, especially in the modern era, a sort of wild card of a pitch. Sometimes it is utterly unhittable, sailing in like a wounded bird, falling and rising and sliding from plane to plane abruptly, jerking always just when it seems most certain that it will hold a plane.
Because it is thrown so slowly, it looks utterly juicy to any hitter following its approach, and one can hardly resist the urge to try to pull it a light year to leftfield. But the ball seems to be guided by a mind of its own, and it ducks under and pulls around the bat as it comes, always refusing to hold a plane--at least when it's thrown well. When it's not thrown well, anyone can hit it a light year to leftfield.
In the modern era, it has become a rule that knuckleballers (knuckleball pitchers) will be dominant one day and horrendous the next. This has to do with how difficult the throwing of this pitch is, and how perfect a grip and release point must be on any given day in order for it to be effective. Remember, if your index finger is a fraction of a centimeter off--light year, leftfield.
For this reason, Tim Wakefield was able to break into baseball appearing unstoppable, and make a huge impact, asserting himself as one of the best in the game over the course of a handful of starts his rookie season. But his effectiveness was shortlived: he lost the feel of the knuckleball, and it took him years to find it again.
In 2001, Steve Sparks, a knuckleballer, led the league in complete games. He used to his advantage a ballpark of formidable proportions, which no doubt saved him from suffering the occasional longball, and the inordinate stamina inherent to throwing the knuckleball: because he can just lob the ball up at 55 - 80 MPH each time, he never wears down his arm, and can pitch almost indefinitely.
You can make millions of dollars in Major League Baseball, if you figure out how to throw this mystery at an unimpressive speed with any regularity. It helps if you have a fastball to complement it, but it's not wholly necessary.