Postgres 9.3中JSON和用户定义类型的比较

I wonder why there is so much fuss about JSON support in Postgres 9.3. What are advantages of JSON over User-defined types (UDTs)? What are pitfalls in using UDTs? Is access to tables with UDTs inefficient? Is ALTER TYPE ADD attribute slow? How are UDTs physically stored by Postgres?I wonder why there is so much fuss about JSON s