Following Jeff Egger, who wrote, in part, "Ban is a fine example which can teach any student of category theory a number of salutary lessons," but asking forgiveness for tooting my own horn, I'd like to point out another one of those lessons -- my old characterization of Banach conjugate spaces as the algebras over the double-dualization monad on {Ban}. Neat mix of Beck Theorem, functional analysis, and more, on pp. 227-240 of: Proc. Conf. Integration, Topology, and Geometry in Linear Spaces, in: Contemporary Mathematics, Volume 2, AMS, Providence, 1980. Might even serve as one student's "individual reading report" project. There's also my even older squib on "Functorial Measure Theory," in pp. 36-49 of: Proc. Conf. Functional Analysis, UC Irvine, 3/28-4/1, 1966, Thompson Book Co., Wash., DC, & Academic Press, London, 1967. This one breathes life into the slogan, "Measures are adjoint to functions." Cheers, -- Fred