Students get bored with the skill-and-drill format rather quickly. Making it into a game will help. I do a Math Marathon with it. For every 60 minutes on iXL, they get a certificate for a slice of pizza. My fourth grade teachers use it in an Olympic-type format, practicing different skills at a time. She too offers an incentive to participate.
Students get bored quickly! I only use it because my principal requires that I do.