It's really situational. There's no better item in racing, unless we're talkin' battle mode, then reds are certainly better when it comes to directly scoring points with them, or hitting someone just after being stunned by a POW. It's really how you use the item that makes it worthwhile.
POWs + curves like in Rainbow Road, or POWing an outer-drifter during a curve are pretty effective. Players are forced to slow down.