Winglets have been widely used in the most aerodynamically efficient class of airplanes, glider planes, for quite some time. They are standard for the last 20 years or so now, even on open class planes where no restriction on wingspan exists. Of course that is purely low speed flying.
The 787 uses a so-called raked wingtip which is in theory more efficient than winglets, but uses a lot more wingspan, for that reason the shorter-range 787-3 was at one time designed with winglets instead (less wingspan for cramped airports). The -3 has vanished for other reasons by now, but i found it interesting that they used basically both methods on a similar wing.