I am a great fan of Whizz.
I registered my DC because she was bored at school and not stimulated with maths but is an extremely capable child. We trialled it and did the test several times, setting the skill levels at different abilities and then chose a level to start at. I actually chose a lower level than her capabilities which turned out to be the best thing I could have done because I found that there were lots of gaps in her knowledge, mainly because the various topics were just never covered at school.
Her maths knowledge is now very comprehensive and she is able to adapt complicated maths because she has been taught the basics step by step of the whole curriculum.
Whizz is a good tutorial package, but I would recommend parents sit with their child to help when required and also see what your child does struggle with and what they are confident in. With using Whizz, your child is taught a variety of methods for each topic and they are taught the methods used in schools rather than methods many parents use.
They are taught across the whole curriculum and cannot bypass topics they dont want to do
All lessons can be replayed which is strongly recommended. The more they practice the more they understand and remember.
Each lesson builds up stage by stage from previous lesson so quite often the child thinks they are doing the same exercise but it is harder and they dont realise.
Only negative I have is you cant choose a topic they are covering at school, you can only replay lessons you have already had. This is fine if the levels are the same but quite often the lessons on Whizz are different stages to what they are being taught at school.
Starting off Whizz at easier levels helps improve the child's confidence when they get 100% for everything so dont see it as a negative if lessons seem easy.
With regular use (90 minutes a week) you can easily get through a lot of lessons and move stages very quickly.
The programme is set in four groups per school year so every single topic has a minimum of 4 lessons for each topic per quarter and many have much more so you get regular revision for every topic in the curriculum