it can be a chore getting them to do the work, but the fact that they go to the centre once a week can help to keep them on track. We were targetted once by an organisation called (I think) the Student Support Centre, which also does extra maths and English. It seemed to be a video based programme. The problem with that one is that the parent is the main motivator, etc. and I think this puts too much pressure on the parent.
With Kumon, you go to the centre once or twice a week, so that the child knows it has to get its work done or it has to explain to the tutor why it hasn't been done (at least that's what I tell my boys).
It's horses for courses, really. If it works and achieves what you want it to achieve, it's worth the money. If it doesn't, it's an expensive way to achieve nothing.