First of all, P&T are wonderfully consummate entertainers, tops in magic acts and comedy. In addition, they have been influential ambassadeurs for healthy skepticism, especially in their Bullshit! series.

I'll agree with some others here that the later seasons were usually less inspiring than the first ones where they brilliantly took on "cold reading", "Feng Shui" and other new age (and old age) nonsense, with notable guests such as James Randi and Christopher Hitchens. Their "Bible" episode was actually a bit tame, though- However, they made up for that with their Vatican episode (season 7), which is still mysteriously eradicated from the network's history: Never re-run or syndicated, unmentioned on the official web page and dropped from the DVD collection.

Still, the Vatican episode can be viewed online and is most heartily recommended.