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.