QUERY ABOUT SECTION 44 AD OF INCOME TAX ACT - Income Tax

Dear All Experts, I want to know about the concept of Section 44AD of Income Tax Act. (i) What are its advantages and disadvantages ? (ii) Is Section 44AD applicable to only consultancy income (excluding pension) ? (iii) Once I opt for Section 44AD , am I legally bound to follow this method for 5 years ? Thanks a lot in anticipation