I sat and passed CT6 in April 2015. It was difficult and had low pass rate (about 45%). Ive had a look at the October 2015 paper and it looks even harder to me! Will be very interesting to see the pass rates when the results are released.
So did I Prior to the exam I felt Apr 2015 was difficult but at least it was mostly numerical but in the Oct 2015 at least 30% derivation and theory, I am not sure how many could derive a posterior of normal in the given time?
I thought October was nicer. Yes there were 2 oddballs at the end - but there were enough marks to give them a good stab. All the rest was really quite straightforward. According to the time it took me to complete each paper - I finished Octobers quicker. Sorry.