Both CRAN and Bioconductor track the number of downloads for their packages over time. You can access these statistics for any of the packages you are interested in. A large number of downloads is an indirect measure of the usefulness of a given package. This can help you decide which among several competing packages is well received by other R programmers.
Guangchuang Yu. dlstats: Download stats of R packages. Available in html format.
An earlier version of this page was published on new.pmean.com.