My son, aged 6, thought this was brilliant. He and his dad spent several very happy afternoons over the summer holidays repeatedly launching the rocket, which cleared the roof of the house several times! Very simple, very easy for a child to manage with adult supervision, and great fun. Highly recommended.
But, if you're getting this, do consider the Horrible Science Explosive Experiments kit which includes this, the volcano, and several (admittedly less brilliant, but still fun) additional 'experiments', for around £15.