Without a doubt, Public Administration is one of the most popular subjects as the optional paper in the UPSC CSE. Even though the level of this paper is at the postgraduate level, many aspirants choose Public Administration because of its role in the actual functioning of the administrative services, police services and some other services to which successful UPSC CSE candidates may be allotted.
Candidates opting for Public Administration should remember that it is a highly conceptual paper and requires an in-depth understanding of the concepts. It is not the kind of subject which you can simply memorise. The resources for study are easily available and the subject is not too difficult to understand. However, the downside of opting for this subject is that you will end up competing with far too many candidates. So, your conceptual clarity and understanding of the various topics in the subject must stand out.
Public Administration will most probably retain its popularity as an optional paper because of its definite viability. It can be an excellent choice for most aspirants.