The short answer—not too well. Looking at the in-depth data from Baseball Info Solutions on all batters in Major League Baseball in 2005, they hit .285 when they put a fastball in play, bit only .221 on curveballs. The slugging percentage difference is even greater—.447 vs. .343.

	MLB Hitters—2005
Ball Hit into Play	Batting Average	Slugging Percentage
Curveballs	.221	.343
Fastballs	.282	.447

Nevertheless, fastballs are thrown seven times more often than curveballs. Why is that? Pretty simple—it's not easy to throw an effective curveball, let alone throwing one for a strike.