We used it in my previous companies and our developers were very happy about it. Previously we were using http://www.amcharts.com/ good product (but we switched because at that time amcharts was flash only - I think), but my dev colleagues told me that Highcharts was more powerful.

P.S. at that time (4-5 years ago) it was possible to use Amcharts for free in commercial app but with reduced features (for example you couldn't handle a click on the graph because in the free edition it would open their site) and showing a link in the bottom of the chart. It seems it follows the same rule now according to their download page, so consider it if you want a good free charting library (I used also amMap and it was ok)