I looked at the website and answered the questionnaire for the two schools our ds's are attending. However, the only questions it asks before you answer the questionnaire is whether you have a child attending the school and whether this is the first time this year you have responded.
If you had a grudge against a school, you could easily answer the questionnaire multiple times using different email addresses. Neither do you actually have to have a child at school as there is no way of checking - you could be a neighbour dis-satisfied with parking arrangements or someone whose child has failed to get into the school - anything really.
The responses are not free text, it is a prepared list of 12 (I think) questions which you mark from excellent to very unhappy, or similar.
And yes, I agree with the poster who described it as Trip Advisor for schools! Quite worrying as under the new no-notice inspections this is how the Ofsted inspectors are going to take account of parent views, given that there will be no opportunity to send out questionnaires in advance to parents.