I was told when I was there that about 75% generally get accepted to Oxbridge - but as dig_the_pony says, this is probably because the people who go on the course go through a selection process, and are typical Oxbridge applicants anyway (i.e they have good grades and are intelligent!), so how much such a course helps is debatable. Basically, the people are on the course because they have a good change of getting into Oxbridge anyway, so of course a high proportion will go on to be successful.
I would say the Eton course is good just for the experience, not necessarily the knowledge or preparation for Oxbridge you might gain (although obviously that's helpful too).
I used the word course a lot in that post.