From the 1990s so many DLs have been designed and implemented around the world. According to T. Sarasevic, there have been a few projects on assessment of DLs. I am examining the literature for some evidences for and against Sarasevic's statement.
There is some work in the area. For a good overview of evaluation in DLs see Fuhr (ref below). DELOS had a work package on evaluation in the mid-00s. See http://dlib.ionio.gr/wp7/
Fuhr, Norbert, et al. "Evaluation of digital libraries." International Journal on Digital Libraries 8.1 (2007): 21-38.