Parked there many times. Well worth the 20 Baht fee.
Check with the attendant if there are any planned funerals at the time you plan to leave the car park . If there is to be a very large service they often lock the car park until the service is over and attendees have left. Sometimes the attendant will show a card with times when exit is not allowed. Got stuck one evening when I wanted to leave at 8pm and gates closed until after 9pm. Just had to spend the time waiting nearby!