What are the advantages of Super-critical Boilers over Sub Critical Boilers ?

Supercritical Boilers are boilers in which the working fluid is above the critical pressure.  At this pressure, water changes into steam without boiling.  This intermediate state is known as a super critical liquid. 
Supercritical boilers are used in Turbine systems. 

The advantages of supercritical boilers over sub critical boilers are

Efficiency
Supercritical boilers are more efficient that sub critical boilers.  The efficiency rating of supercritical boilers is in the range of 32 - 38 % while that of ordinary boilers is in the range of 32% - 38%.

Reduced Operating Costs
As the efficiency increases, there is a natural reduction in fuel costs which translates into reduced operating costs

Lower Emissions
Due to less fuel being burnt, there are lower emissions.

Higher Initial Costs
The downside is that super-critical boilers have higher initial costs as the boiler and the systems have to be designed to withstand higher pressures. 

Advanced Water Chemistry
Supercritical boilers require very pure water.  Even small levels of impurities can cause deposits on the turbine blades.