For about the first quarter of Derren Brown's show I felt that his act was very impressive, but nothing that I hadn't already seen him do on the television before. I was happy to be there but didn't feel that being in the actual room in which he was performing was enhancing the experience. However, towards the end of the first half, the pace really picked up and he started to do some quite startling things... by the time the interval came, I was quite hooked.
He (understandably) requested that the details of his act be kept secret so as not to ruin it for other people, so I probably shouldn't say too much about the details of what he did, but to say that even for committed fans, there will be fresh and new ideas in there and the show certainly ends with impact!