What are the advantages and disadvantages of using catalytic converters?

What are the advantages and disadvantages of using catalytic converters?
The advantages of CC's are because of them, levels of CO2, carbon monoxide and hydrocarbons in the atmosphere drop alot. Also, the introduction of catalytic converters led to the introduction of unleaded gasoline, since catalytic converters do not work with leaded gasoline. As a also caused atmospheric levels of lead particulates to drop.

The disadvantages, however, are that they are very expensive to get/replace. Also, because carbon dioxide is a product of the reaction that takes place inside catalytic converters, widespread use of catalytic converters has also contributed to increased atmosphere levels of carbon dioxide.