OCP is a dream. The goal is to be able to work all the land while having minimal overlap. This allows for some very good cities in the late game, but frankly, the game is over for ages before it becomes really important. Indeed the best settling options have almost always been about getting the benefits of the good tiles soonest. So having to wait for your supercities to grow to size 20 (and having the health and happiness caps rise that much) is too long. Moreover you will be getting many junk tiles in your cities doing OCP. Besides it is quite often impossible to do OCP as lakes and mountains come in the way of placing your city in the exact right spot. And you easily miss out on the important river or coast access.
I actually do think OCP is better suited for Civ IV than for Civ III, as it is a relatively loose grid which is neccessary to offset the number of city maintenance (thus ICS becomes less of an option).
However, I do think it is a good training to play one game following OCP as it gives a feeling for a reasonable distance of cities. (Much wider and you lose tiles, closer in gives extra overlap). Besides having a complete continent in (almost) OCP just looks great (if you like an ordered empire).