I've used Weka, and RapidMiner once. As I recall, RapidMiner seemed to be general purpose, but lots of posts were for using it for data mining stock data to build a model.

I think it would be interesting to see breakdowns of different software, and where they are used. Often times it seems to me that people just use the tools their peers and co-workers use, and people tend learn to like whatever they use most.