No matter what your favourite statistical package is, you'll find this post by Robert Muenchen highly informative.
Robert concludes that:
"By most of the measures discussed here, R is competing well with the commercial software vendors. However, I advise not over generalizing from this data. SAS and SPSS continue to dominate the corporate world and Stata is doing quite well in the scholarly arena. Each of these packages is dominant in one market or another."
© 2012, David E. Giles