We haven't tried IXL, but we did Dreambox for a while with mixed results.

DD7 liked it at first, and the activities do teach number sense in an interesting way. The graphics and story lines were a big draw. She quit when 1) all of the games and story line movies she liked to spend her incentive tokens on were removed abruptly in January, and 2) she got bored and frustrated with being assessed on her mouse skills while using virtual manipulatives and clicking on the hundreds chart. She was approaching the end of 2nd grade and knew the material but couldn't make it through the activities.

The mouse skills wouldn't have been an issue if she could have used the iPad version, but her iPad is too old. Something to keep in mind.