Questionnaires; What's good and bad about it?

Questionnaires are Primary Research -Can be achieved via Telephones, post or Face to face -Given to connected and potential customers -Is usually Cheap and can show good Quality Data [if a certain size is reached] -Data is usually Quantitative, making it easy to see Bad; -Has to be well-made, otherwise the information won't be reliable -Reply rates are low, noticeable on Telephones and Post -Could be expensive and takes lot of time to gain a true sum of the majority's views