We did it briefly for both dss, but they were older - I think it is probably much better for younger ones, so you're probably at a good age.
It is a big commitment and you have to be prepared to be heavily involved as well. They have a worksheet that has to be done every day - including holidays and Christmas day. It should only take 10 minutes, but then you have to mark it as well. For us it became a daily torture to do this!
They will assess your DD's current level and then will give her worksheets that are deliberately below her level. The idea of this is that she will find them easy to do, rather than strugging, and they will only move her on when she can cope with it.
It will be very basic arithmatic, and is highly repetitive - the aim, as I understand it, is to help them learn their numbers/tables by this kind of repetition, so that basic sums just come to them instantly.
They use lots of stickers/rewards to encourage development.
I think, on the whole, it is a good idea - and if you start now there is a chance that your DD will find it fun - not a real chore like my sons did!