Our school tried Front Row for a year, and the kids seemed to like it, except when it gave wrong answers or didn't count a correct answer, which irritated them to no end. From what I recall, it also didn't flow as easily to the next topic, and it topped out at a certain point in depth (I'm thinking ours topped at grade 5 at the time).
DC have enjoyed the reward system of Khan more, and it takes them into territory they might not know and have to hit the 'I don't know this yet' button once in a blue moon. Front Row seemed more like what I've read about IXL, though it did have instruction videos, it came across as more practice oriented.
A benefit of Front Row is that the teacher could track the progress of the students, though they could also login at home to work during free time.
