I'd be interested in peoples experiences, especially in GCSE exams.
I've always been very keen on DD, who is dyslexic, reading herself as much as possible, and she now has a reading age above her chronological age 
. However, she misread a word in an assessment today and wrote an awful lot about the wrong thing, before realising her mistake. 
They're not cheap so I'd love to hear real life experiences before I invest.