Another good article by Grant Ingersoll on Mahout. I used Mahout on a customer project last year when it was not yet a complete machine learning system layered on Hadoop. Looking at Table 1. in this article, many of the previous gaps have been implemented. BTW, the book Mahout in Action is a good guide but the new MEAP released last week does not cover some of the new features, which is OK. Also, Grant has been working on "Taming Text" for a while, but a new MEAP has not been released in a year or two - I would bet that his energies have been focused on extending and using Mahout.