I am now putting my fourth and final son through the process of application for a Grammar School place.
My first did not make it as I did not have a clue about preparation and a website like this would have been a big help the first time round, as would not having four boys!
But now I am a seasoned campaigner and I can tell you, having even spoken directly to one examiner, that practice papers add 15% to the average child's exam day performance. So its well worth the effort - however I would also advise against just buying everything in sight - you can always "borrow" from another child who has just completed their attempt.
Hope this helps, Sally