It will not positively affect your chances of getting into medicine. I doubt it makes a difference when adding it into your personal statement as all it shows is that you had the money to go to an expensive course which bombards you with medical facts. The stethoscope they give you isn't useful. The information they give you can be obtained for FREE online. You get zero medical experience and you are literally sat in a hall of 800 students listening to lecturers, who try to make it fun, some succeed.
This really isn't necessary, and for those who can't afford going, you are not missing out on much, For those who can and want a bit of fun, then you might as well go, as you meet a lot of like-minded people who all are in the same boat.
You also get a taste on university life and how it feels to be living in halls which is good.