The Citation X was already a text book study in aerodynamics, and Cessna is writing another chapter with the addition of winglets. They seem like overkill on an aircraft already so highly swept and area ruled that, at Mach 0.92, it is the fastest commercial aircraft flying today.
No other Cessna's have them, but the company says winglets should improve the Citation X's hot-and-high take-off and climb performance and high-altitude speed and range. Maybe it's another example, after the Dassault Falcon, of a bizjet manufacturer finally accepting that winglets can look good AND work.
It's actually a little too early to say if they do work as advertised, but Cessna is supporting efforts by Wichita-based WInglet Technology to get supplemental type certification for its elliptical winglets for retrofit on to the Citation X. Elliptical refers to the curvature of winglet, by the way, which is designed to minimise the induced drag.