The power of R (speaking as a very heavy user who has deployed it in multiple production environments and been using it for 5 years) is that it makes it very fast, easy, and natural to do statistics. It also has the nicest data structure I've ever seen for manipulating table data, called a data frame -- I'll elaborate, if anybody cares. In addition, it encourages people to create packages to extend the functionality. There are extant packages to do almost every analysis you can think of -- time series, kmeans, other clustering techniques, cox-box style analyses, regular maximum likelihood style GLM, hierarchical regression, HB, etc. Further, the amount of knowledge and the open source nature of the language, base, and packages encourage additional development and widespread adoption.
See:
<a href="http://cran.r-project.org/" rel="nofollow">http://cran.r-project.org/</a>
and
<a href="http://cran.r-project.org/web/views/" rel="nofollow">http://cran.r-project.org/web/views/</a>
^ is task views. Explore it -- it's well worth your time.<p>The downsides are, well, it's slow for large data sets and debugging can be difficult. But as a desktop / rapid development platform for statistics it is without peer, IMO.<p>ps -- unlike Matlab, which often costs thousands of dollars, and the Statistics Toolbox, more thousands, R is free. This is pretty important on its own -- instead of $5k per server and workstation and home pc, install it on any linux, Mac, or windows box you have and get to work for $0.00.