Mahout is a great platform, but the real challenge is defining your learning problems, preparing data sets and choosing right algorithms.
Once you are clear as to what you actually want to accomplish chances are you are going to need some kind of significantly modified or hybrid algorithm. Packages like Mahout could help get started, but it is kinda funny that even quite a few examples in this article do not demonstrate actually good algorithm performance, like this one -
Correctly Classified Instances : 41523 61.9219%
Incorrectly Classified Instances : 25534 38.0781%
Total Classified Instances : 67057
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Confusion Matrix
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a b c d e f ><--Classified as
190440 12 1069 0 0 | 20125 a= cocoon_apache_org_dev
2066 0 1 477 0 0 | 2544 b= cocoon_apache_org_docs
165480 2370 704 0 0 | 19622 c= cocoon_apache_org_users
58 0 0 201090 0 | 20167 d= commons_apache_org_dev
147 0 1 4451 0 0 | 4599 e= commons_apache_org_user