I developed the R package effsize, one and half years ago, in July 2013.
It is a package for efficient effect size computation. The computation algorithms have been optimized to allow efficient computation even with very large data sets. It contains the functions to compute the standardized effect sizes for experiments (Cohen d, Hedges g, Cliff delta, Vargha and Delaney A).
Today I looked at the download statistics from http://cran-logs.rstudio.com/. The mirror linked to the most famous R IDE, R Studio. The statistics refer to only one out of 100+ mirrors of the R project.
The number of unique IP, which downloaded is shown in the following figure:
What I saw represents a success for me,during the last year an average 311 distinct user per month installed the package.
Of course that figure is nothing, when compare to very famous packages, such as ggplot2 or plyr, that count monthly downloads in the order of 15000, i.e. 50 times more than mine.
Nevertheless for being a small, very limited, package I consider it a success. I take it as a stimulus to devote my time to this small piece of code.