The theatre is justly famous. When I was there, several children were testing the "you can hear a coin drop from the top tier" claim; it really is true! More interesting to me, however, was the sanctuary itself, with its hotel, baths, cafeteria (later transformed into a theater by the Romans; why they needed one when they had the great theatre a few hundred yards away is a mystery to me), tholos, and temples. Extensive reconstruction work was under way during my visit in 2006. Far from being a nuisance, it was fascinating to see the care and diligence with which the Greek people and their benefactors work on the ancient sites. This site was one of the most comprehensively sign-posted and hence easiest to understand (though I suspect you could study it for many years and never get it all) of all the places in Greece we visited.