Owen is correct. I always do recommend R.
In fact, I also use R quite a lot. It is really valuable as a reference implementation or for exploratory use. For production, I very much prefer Mahout.
Another interesting related project is Vowpal Wabbit. Their focus is even more narrow than Mahout's and it is much harder to integrate VW models into a working system. See
http://hunch.net/~vw/ for more information.