I found Weka very easy and flexible to use and learn. The visualizations are simple to implement and understand. Incorporating several difficult clustering and classification algorithms becomes fairly simple.

Weka represents a high-level solution for software maturity and brilliant results that have captured the attention of a vast scientific community. Weka contains an advanced collection of machine learning algorithms and preprocessing techniques. Weka was designed to test different existing methodologies on data sets, quickly and flexibly; in fact, it provides support for the entire data mining experimentation process: the preparation of input data, the statistical evaluation of learning schemes, the graphic visualization of input data and the learning outcomes.

I like how easy is to use Weka, and the flexibility it provides by accepting multiple formats from multiple sources when reading data. The data can come from files or databases. From an individual stand point, Weka is free and open source, it can be extended and use in multiple programming languages.