I used to use R all the time until I got into Python, NumPy, and SciPy.
In general, however, R has much more sophisticated packages for statistics (I don’t think there’s too much argument here). Unfortunately, R, in my opinion, is a cumbersome language for string manipulations or more general programming (though it is fairly capable).
You can get the best of both worlds with RPy.
I recently wanted to calculate the rank of a matrix via a QR decomposition as an alternative to computing the SVD. I haven’t yet seen code to do this in numpy. R, on the other hand, computes this automatically in its ‘qr’ function. To get this running in my numpy/scipy code, I didn’t even have to worry much about matrix format conversions between the languages. After a couple of ‘imports’ and within 5 minutes I calculated the rank in Python via the R code:
rqr = robjects.r[‘qr’]