Just from the opinions that are newsworthy, it seems about 75% for Kennedy or O'Connor would be about right. That would make 90% a reasonable number as a significant improvement over O'Connor and would have to be considered Roberts a great pick. And $100 donation to FR sounds good.
OK, for the term before last this is what I came up with. Justice O'Connor agreed in full with either Justice Thomas or Justice Scalia in 70.12% of cases; Justice Kennedy agreed in full with either Justice Thomas or Justice Scalia in 74.68% of cases. Justice O'Connor did not take part in two cases, and I did not count the Pledge of Allegiance case because Justice Scalia did not take part.
So! My personal expectation is that Justice Roberts will end up somewhere to the right of Justice Kennedy and to the left of Justice Rehnquist. I will be very surprised if in 90%+ of rulings Justice Roberts reaches agreement in full with either one of Justice Thomas or Justice Scalia. $100 donation to FR is our bet then. Let the games begin!
We shall meet again by June 2006. =)