I use it mostly for exploratory data analysis. I'm pretty quick with making plots in it (I've used it since v1.2), so I tend to use it for reading in a data set, and them playing around with it. It can handle datasets in the hundreds of thousands of points (still in memory) pretty quickly. You can plot 200k points a series without it slowing down.

On those rare occasions I need to solve equations or do some calculus, there's nothing better.